We are playing 3.5. The party will most likely consists of 4-6 characters -- a 3rd level Halfling rogue; 1 or 2 second level H-E druids (+ companions); 1 or 2 third level clerics (one is Healing/Good Human, the other is Luck H-E); and a second level H/E Ranger. They are in a swamp that is foggy and rainy most of the time. They are simply trying to traverse the swamp to get to a forest on the otherside. I do have a hag in her lair for later on in the adventure--she is a key plot device. I also have ruins from the lizardfolk civilization that once lived there. The previous post by Galladian made me think of creating a lizardfolk hermit--the last of his kind in the area that could be an ally through portions of the swamp. Also, I was thinking of have something like bugbites that cause penalties unless the druids/ranger can make natural "insect repllent" from the "local ingredients" using their nature knowledge and survival skills. I like the flooding, getting lost, and hornets nest ideas as well.
Combat encounters include bullywugs, stirges, crocs, snakes, a shipwreck with combination harpy/lacedons, ruined lizardfolk shrine with a grey ooze, special chimera through a rocky pass through the swamp. Maybe a very scary shambling mound they can simply avoid.