Around Denver, leaves are turning, but colors aren't at their peak yet, but you should be able to catch some nice colors at higher elevation, weather permitting. Sometimes as you pass Georgetown (one of a number of scenic mountain towns along I-70), you can see one of their Shay steam locomotives in operation. Gobs of photo opportunities there alone both as spectator and passenger.
Englewood Camera ought to be one of the better places to find secondhand and film-related items. Denver Pro Photo tended to focus more on the film + processing itself. Have not been to either in ages as they're pretty far out of my way.
Denver Botanic Gardens has a large enclosed space housing tropical vegetation. Sometimes in the middle of winter, I wish I could live there.