Psychonauts

• Up until a certain point, you'll be able to backtrack and revisit all levels. The game will warn you at that point, so you can do all your backtracking then. I'm pretty sure that there's nothing you'll be able to permanently miss. It is easier to get a lot of things in the camp during the daytime, though, and once night starts, it doesn't end.

• You want to get the dowsing rod, then the web vacuum from the shop before anything else. The former makes getting money much quicker, while the latter helps completion.