When you say "other kinds of adventures" . . .
In Gygax's AD&D, what is the goal of adventuring? It is set out by Gygax in his PHB and DMG:
PHB pp 18, 106:
Character class refers to the profession of the player character. The approach you wish to take to the game, how you believe you can most successfully meet the challenges which it poses, and which role you desire to play are dictated by character class (or multi-class). . . . It is up to you to select what class you desire your character to be. . . .
Experience is the measure of a character's ability in his or her chosen profession, the character's class. Each player character begins the campaign at 1st level with no experience points accumulated. Thereafter, as he or she completes adventures and returns to an established base of operations, the Dungeon Master will award experience points to the character for treasure gained and opponents captured or slain and for solving or overcoming problems through professional means.. . .
As a rule, one point of experience will be awarded for one gold piece gained by a character, with copper pieces, silver pieces, electrum pieces, platinum pieces, gems, jewelry, and like treasure being converted to a gold piece value. Magic items gained and retained have only a low experience point value, for they benefit the character through their use. Magic items gained and sold immediately are treated as gold pieces, the selling price bringing an award in experience on the stated one for one basis. Experience points awarded for treasure gained - monetary or magical - are modified downward if the guardian of the treasure (whether a monster, device, or obstacle, such as a secret door or maze) was generally weaker than the character who overcame it. . . .
Monsters captured or slain always bring a full experience point award. Captured monsters ransomed or sold bring a gold piece: experience point ratio award. Monsters slain gain a set point award. Low hit point/dice monsters have a low experience point amount. Monsters with high hit point/dice have large experience point awards. Special abilities such as magic resistance, spell capability, gaze or breath weapons, regeneration, and the like also increase experience points amounts.
Finally, clerics' major aims are to use their spell abilities to aid during any given encounter, fighters aim to engage in combat, magic-users aim to cast spells, thieves aim to make gain by stealth, and monks aim to use their unusual talents to come to successful ends. If characters gain treasure by pursuit of their major aims, then they are generally entitled to a full share of earned experience points awarded by the DM.
DMG p 86:
Consider the natural functions of each class of character. Consider also the professed alignment of each character. Briefly assess the performance of each character after an adventure. Did he or she perform basically in the character of his or her class? Were his or her actions in keeping with his or her professed alignment? . . .
Clerics who refuse to help and heal or do not remain faithful to their deity, fighters who hang bock from combat or attempt to steal, or fail to boldly lead, magic-users who seek to engage in melee or ignore magic items they could employ in crucial situations, thieves who boldly engage in frontal attacks or refrain from acquisition of an extra bit of treasure when the opportunity presents itself, "cautious" characters who do not pull their own weight - these are all clear examples of [poor performance.]
If you read that part of the DMG a bit more closely, you'll see the actual rules are a bit wonky - in the PHB, the idea of a class's "major aims"/"natural function" is meant to feed into the XP awarded, whereas in the DMG it becomes an adjustment to the training time required (and hence expenditure required) to actually gain a level. But the
ethos is clear: the goal of play is to perform your class functions, consistently with your alignment, so as to acquire treasure, thereby earning XP, thereby improving your level and hence your class functions, therefore enabling the acquisition of bigger treasures that are more difficult to get.
The DMG has instructions to the GM, on how to prepare to run the game, that reflect this:
DMG pp 86-7, 96:
The milieu for initial adventures should be kept to a size commensurate with the needs of campaign participants - your available time as compared with the demands of the players. This will typically result in your giving them a brief background, placing them in a settlement, and stating that they should prepare themselves to find and explore the dungeon/ruin they know is nearby. As background you inform them that they are from some nearby place where they were apprentices learning their respective professions, that they met by chance in an inn or tavern and resolved to journey together to seek their fortunes in the dangerous environment, and that, beyond the knowledge common to the area (speech, alignments, races, and the like), they know nothing of the world. . . .
Assume that you have assembled a group of players. Each has created a character, determined his or her race and profession, and spent some time carefully equipping these neophyte adventurers with everything that the limited funds available could purchase. Your participants are now eagerly awaiting instructions from you as to how to find the place they are to seek their fortunes in. You inform them that there is a rumor in the village that something strange and terrible lurks in the abandoned monastery not far from the place. . . .
You inform them that after about a two mile trek along a seldom-used road, they come to the edge of a fen. A narrow causeway leads out to a low mound upon which stand the walls and buildings of the deserted monastery. One of the players inquires if the mound appears to be travelled, and you inform the party that only a very faint path is discernible - as if any traffic is light and infrequent. Somewhat reassured, another player asks if anything else is apparent. You describe the general bleakness of the bag, with little to relieve the view save a few clumps of brush and tamarack sprouting here and there (probably on bits of higher ground) and a fairly dense cluster of the same type of growth approximately a half mile beyond the abandoned place. Thus, the party has only one place to go along the causeway - if they wish to adventure. The leading member of the group (whether appointed or self-elected, it makes no difference) orders that the party should proceed along the raised pathway to the monastery, and the real adventure begins.
There is an example dungeon, and example of dungeon play, rules for placing treasure (which refer overwhelmingly to its placement in dungeons), and the action resolution rules deal primarily with combat and with dealing with doors and traps in dungeons. The magical abilities available in the game are focused primarily on dungeon exploration (including detection magic that finds traps and treasure, and other magic that bypasses pits, doors and walls). And the advice to players on Successful Adventures (PHB pp 107-8) is all about how to prepare for and undertake a successful dungeon expedition (there is one paragraph, the second-last on p 108, that deals with wilderness exploration and city adventures).
Whether or not one enjoys this game, it is a coherent game design. The fantasy trappings, the scenario goals, etc are all fig leaves or "lampshades" for dungeon exploration play.
Take away gp for XP, and then the questions arise like: why does the play of the game focus on dungeon exploration, and acquisition of treasure? Without that progression rule, there is nothing for the fantasy trappings and scenario goals to lampshade. Are the PCs just greedy psychopaths?
The most natural answer to this is to take away dungeon exploration and acquisition of treasure as the focuses of play. So now, what
is the focus of play. The answer that emerges in the early-to-mid 80s and is consolidated by AD&D 2nd ed, is
whatever the GM says it is.
But if the game is no longer about dungeon exploration and acquisition of treasure, then why do we have all this magic that is focused primarily on that stuff? And what are the rules for resolving non-dungeon actions, like wooing or chariot-racing or rousing the villagers to resist bandits or the myriad other things that actually make sense for fantasy protagonists? The answer that emerges is
whatever the GM says they are.
And if the game no longer uses a dungeon map-and-key to structure the way in which PCs encounter challenges, and in which players can make choices about which challenges their PCs encounter, then how is pacing and scene-framing to be handled? The answer that emerges is
as the GM decides.
So if you want to have other kinds of adventures, but want a game that is structured around something different from
whatever the GM says and decides, then you will need a system other than the one provided in the AD&D volumes. My personal favourite is Burning Wheel, but there are plenty of others out there!