I was going to suggest some environmental hazards, but that thread linked above does a much better job than my meager spur of the moment ideas would. Give it a read, it's nifty stuff.

However the point I was going to make still stands - encounters aren't all about the enemies. Use the swamp itself as a character - swamps are creepy and nasty, make them feel like the place itself is out to get them. If you've read Streams of Silver (the second R.A. Salvatore F.R. book) there's a lengthy passage in there that evokes just this kind of horror. The heroes get so exhausted that they have to resort to using logs as makeshift boats to sleep on while they take turns hacking up the awful water trolls that pop out randomly, and by the end none of them can even walk anymore.

Have enemies toy with them. Crafty swamp dwelling critters wouldn't rush in right away, they'll watch for a while, try and attack the PC's when they're at their weakest. Just make sure the PC's know they're being watched, and that they could get attacked at any moment.

Other things for inspiration: Fire swamp sequence from The Princess Bride, Dagobah scenes in Empire Strikes Back, anything set in the southern bayou (The Rescuers, obscure animated movie from the 80's, comes to mind).