A good place to start might be the pet store (any place with a decent tropical fish section) and see what plastic plants they have - these make for simple jungle (quick and easy - but don't look great. Better is anything that mimics a mangrove root - I picked three up at a pet store for about £15 (don't know the exchange rate sorry
). On the board they look like the roots/base of giant trees through which your guys can move, hide in etc. - I'll get a pic up when I get a chance.
Another idea for alien rock/fungal growths/plants is to spray foam gap filler onto a shaped foamboard base, you can then paint or spray it to represent a range of deathworld kinds of terrain - this is really easy to do. The good thing about this stuff is that you can cut flat sections into it to create loads of different levels, fix platforms etc and still get an organic look - good for a jungle outpost perhaps.
Just a few ideas
Laney
edit: Found this link when I was looking for block insulation terrain... contains some nice ideas
http://www.tripleacegames.com/Downloads/TerrainBuilding/TT_01_Jungle.pdf