There is no 'outdoors'. It only exists in your mind.
Everywhere is 'in the dungeon'. You simply dress it different so that that secret is hidden from the players (or at least, you dress it well enough that they can allow themselves to forget the secret).
Your 'outdoor encounters' simply happen in a larger room. This is often good, since all that extra space lets you run more monsters, bigger monsters, and faster moving monsters.
The main thing you really have to worry about in 'outdoor rooms' is the arbitrarily high ceilings. Make sure that your PC's can't dominate the encounter simply by going up, and you are golden.
To begin with, it helps if you've ever been in the real outdoors. Draw upon that for inspiration. Make the 'outdoor rooms' look like the real outdoors in all of its nastily complex glory. Or at the very least, make the encounter area as complex as the green of a championship golf course.
A typical temperate climate encounter in the wilderness should have some water hazards, some difficult terrain, some elevation changes, some sloped areas, some obstacles, and some things that provide cover (trees).
Now, decide what sort of encounter you want to have. Is the monster naturally hostile? What does the monster want? Is it going to try to communicate with the PCs? Is it a meeting encounter, where the monster just happens to wander into the same area as the PC's? Is the monster otherwise engaged in some other task when the PC's encounter it? Is the monster actively stalking the PC party? Is the monster lying in ambush for the PC party?
Meeting encounters: These should probably begin at fairly large distances unless the monster is naturally very stealthy. Even if the local vegetation is so thick that the players can't see more than a few yards, they may hear the beast crashing through the foilage over a long distance. What is it that they are hearing? Can the PC's correctly position themselves for an ambush, or will the creature break into view in some place unexpected? Should they try to run away before whatever is coming gets here?
In the case of stealthy creatures in dense foilage, both parties could be surprised. Ambush hunters will probably have the first instinct to run away and hide, and then later stalking the PC's. This could lead to a cat and mouse game pitting the two sides senses and wits against each other.
In the case of a more open encounter, say in a savannah region or a meadow, the initial encounter might be at 400 yards or more - quite beyond either sides attack range. Who will chase who? There is a good chance both sides are uninterested in the other one? There could simply be a stand off before one side or the other decides to give way.
Engaged In Another Task: One of the best ways to give life to a campaign is have the players stumble over a monster that is busy living its life quite independent of the existance of the PC's. The monster could be hunting something and have no interest in the PC's - finding them the sort of unwelcome distraction they may find it. The monster could have successfully killed something, and is now enjoying the spoils (whatever that means for a monster of its intelligence). The monster may be engaged in a courtship ritual with a nearby potential mate - lots of strange very noisy screaching and yelling. If the monster is intelligent, it could be observing some religious rite, celebration, or leisure activity ('You find a warband of orcs, relaxing in the hot of the evening in a swimming hole'). If the monster is a tool user, it could be engaged in harvesting useful resources - stone, wicker, wood, whatever ('You've come across an orc logging camp'). The interesting thing about encounters of this sort is you are often turning the advantage and the initiative over to the PC's. You can put much more orcs into an orc logging camp than you could into well prepared orc ambush, and still make for a reasonable and fair encounter. You are also opening up RP possibilities.
Stalking the Party: Some monsters are likely to find the PC's intrusions interesting and most likely to discover where the PC's have been before stumbling on the PC's. They'll find old campsites. They'll pick up the PC's trail. This produces an interesting cat and mouse game. If the monster is stealthy, it may want to hang back and wait until the PC's are at a disadvantage - asleep, fighting something else or just finished fighting something else (the RBDM in me), it's dark, the terrain favors it, the party splits up, someone in the party takes a bathroom break, etc. Of course, the longer it waits for the perfect oppurtunity, the more likely it is that the party will realize its being followed. And of course, the monster may realize tha the party realizes its being followed, and you may end up with both sides trying to catch the other in an ambush.
Other monsters may want the players to know they are on the chase. Wolves may howl. Jackerwocks may burble. Players may freak. Good times.
Ambush: For a great many DM's, particularly novice DM's, this is the only way they ever introduce a monster. It always starts with, 'Roll for initiative', and if your lucky, 'May I have a spot check?'. Still, there is something tense and exciting about an ambush, there is often good reason for one (its an ambush predator, its the only way to make a weak monster viable, there is a chokepoint that is being defended, etc.). An ambush represents an excellent oppurtunity to show case the best a monster has to offer tactically and use really interesting terrain. So especially if the monster isn't above party CR (or whatever 4e uses to estimate these things), by all means make the terrain as favorable to the monsters particular skills as is reasonable.
So, for example, suppose you want to lay an ambush for the party and you plan to use hobgoblin fighters of fourth level or less (and mostly less), and the party is like ninth level or something crazy like that. First of all, don't group the hobgoblins in one close together pileup that can be taken out by a fireball and a charge-full attack barage. Instead, set the ambush in a small valley, arm the hobgoblins with longbows, and spread them out over an area say 400 feet in diameter. Have arrows sailing in from all over. Have them spread out enough that a fireball won't hit more than a couple. Give them fighting positions so that they have cover. If the hobgoblins need a 20 to hit, try to make it so that the PC's do too. Scatter a few simple snare or pit traps about the area - the sort that would make your rogue laugh at how hopelessly primitive and obvious they are - but which will still probably catch your distracted charging fighter as he stumbles up the hillside up steep embankments and through tangles of blackberry bushes.
Remember, it's not 'outdoors', it's just (potentially) a much larger dungeon chamber than you are used to designing. But it should have 'encounter traps', positions of tactical importance (high ground, places to take cover, places to hide, good exits), obstacles to hurdle or climb over, and all the rest just like the dungeon rooms you design.
I'm willing to play 'Iron Wandering Monster DM'. Give me a monster, I'll give you an encounter.