could you post some artwork or reference photos for us? because there's so many different kinds of forest/cave etc.
Also do you have pictures of the characters? their style could be a big influence on the sets.

heres the pictures remember i did these in 10 minutes and all are going to be remade and a lot better any way here they are

for cave level

for the forest and field levels

for castle level and battle field


also several more will be made sorry for the terrible quality 

Wow, those are little puppets!
If you find the making of videos from "The Neverhood" PC game, you can see they make structures from wood and board, and coat them in melted Van Aken, now this obviously uses up a lot of clay, and it's pretty dangerous, but you might want to try it for walls, and the ground, etc. I think that look would fit your characters.

Take a look at TerraGenesis. It's a wargaming site, but has lots of ideas for scenery and suchlike.

m_) are you sure you want to try that?

forget cooking again there., if you do.