What's the difference between an adventure on one hand and an opportunity/situation on the other?
If you get a chance, hop on over to Wizard's site and check out James Wyatt's Dungeoncraft articles. I believe that his early articles are still free and the latter ones are part of DDI. He does an excellent job of explaining how he goes about creating a DnD campaign. Methods, ideas, etc.
In his article on 2/13/08, he discusses concepts like quests, and long-term goals. In his next article on 4/30/08, he fleshes those ideas out into a loose outline of how the campaign will flow over the course of thirty levels. To me, this is adventure design.
Where I differ from this method is that I do no plan out quests, long-term goals, or campaign outlines. In their place, I create situations that the characters can pursue or ignore as they see fit. Imagine, if you will, a blank piece of paper. On it, I place a couple of towns, a horde of orcs, an undead-filled graveyard, a lich, a dragon, etc., etc. I do not know which the PCs will choose to pursue, but when they pick a direction, I then begin filling in things to enhance this adventure. They may follow multiple paths. they make connections between things that I didn't anticipate. To me, part of the fun comes from exploring the unknown.
There are positives and negatives to both ways. Part of it comes from when do you, the DM, want to do the work for the campaign, and how do you want to handle it when the PCs veer of path. In my method, I do a little bit of work at a time rather than a lot of work at the beginning. However, I also have to "wing it" more often because I do not have a lot of detail planned out. If that is something you are comfortable with, great, give it a try - I like it. If not, check out James's articles, they are a great description of how to do it another way. It really comes down to what you and your players are comfortable with.
