Yes, you can decorate a door there, but unless it's around a holiday - I don't really see many door/window decorations. The "rooms" themselves at Pop are going to be same - but different building have different theming on the outside and can be closer/farther from the bus stop/lobby/lake/pool, etc. So - you really should look at a map of the resort - it's available on the Disney website.
As for a special idea - maybe book a tour? Wild Africa Trek? Or if that's too much extra cost - suggest a theme of "let's do this" every day - or try something new. It could be a silly game of how many pressed pennies you can collect (or maybe try to collect 40!). Having a drink at each of the resorts on the monorail line one night. Doing the Kidcot activity (as adults) in EPCOT. Trying SOTMK. Get ice cream as many times as you can. Just something "unique" to this trip that might not be part of a normal trip. Or have a list of 40 things to do while on the trip and see if you can cross them all off - stuff like - ride Mission Space, go to Tom Sawyer's Island, get a hot dog at Casey's. Wear the Anniversary button! Plan to dress up one night for one special meal - maybe at a place you haven't been before.