I'm a fairly new DM (I've only had one session as dm), and I find that you shouldn't try to over complicate things. If your players know that you're a new DM, then they probably won't expect something too complicated. As for combat and puzzles, add simple traps, like small pitfalls and tripwires that knock down branches, nuts, and the like. For puzzles, don't make something complicated, as the enemies are squirrels, they can't make something too complex, so make puzzles that are something like this: Place weights on platforms that have certain markings on them (claw marks to signify how many nuts needed) to open a passage leading to a new area. Combat, as you said, they should just fight swarms of squirrels that ambush them. Since the weresquirrel is an elf, add some simple magic spells to the fights (look at low level Druid spells for examples that may fit).
Edit: To get players to enjoy scenery, add things they can do with it, or they will probably just ignore it. Related: How to discourage players from rushing?