10th level Druid in the Underdark: SUMMON EARTH ELEMENTALS! See if you can get your DM to allow you to have an Elemental Companion (Earth, of course) that's exactly as powerful as whatever animal you would normally have. (Animal <-> Elemental equivalence can be estimated by looking at what the Summon Nature's Ally spells can give you at various levels.) If you can't, you can still use Earth Elementals to good effect: they can flank foes who are up against a wall, after all!
You still control the environment, even if it's not a standard environment:
Stone Spells: Stone Shape, Spike Stones, Transmute Rock <-> Mud, and soon Wall of Stone to control the most basic physical environment
Hallow: Put a Quaal's Feather Token: Tree here, so there's an Oak tree within the Hallow'd zone, then you can easily make your escape using another QFT: Tree and the Tree Stride (or Transport Via Plants) spell. Having a Hallow'd base camp is useful for a variety of reasons, which should be obvious. I'd put a Protection from Scrying in the Hallow if possible.
Information Getting / Preventing: Control Plants (deliberately includes fungi), Obscuring Mist + Blindsight, Pass Without Trace, Scrying, and most of all..
Commune With Nature -- even more valuable when the map is 3d.
Rusting Grasp -- Hi, Mr. Iron Golem! Bye, Mr. Iron Golem!
Giant Vermin -- the Underdark has some, ah, interesting vermin.
Druids are cool.
-- Nifft