I spent my last five hours just wandering around the map, gathering various collectibles. It's a relaxing activity, combing through the wilderness, peeking in corners, (occasionally) consulting a guide to see where to go next. It's bringing order to a chaotic system. Like sorting through my magic cards or organizing the pieces of a board game (two activities that I really enjoy that my wife simply does not understand).
In addition, Psychonauts does something interesting with some of its collectibles. Whenever you recover one of your fellow campers' brains (there's a really weird story there, involving kidnapping and brain extraction), it shows you a little scene where Raz and the rescued kid reconnect. And in the process you learn a little something about everyone at the camp. It's a great way to get some more mileage out of some thinly-sketched characters.
I also really enjoyed going around camp and using my clairvoyance power to see Raz through the eyes of the other children. He alternately appeared as an insect to the girls that ignored him, a giant ear to kid who talked all the time, and most mysteriously, nothing but a hat to Doogan, the kid who could blow stuff up with his mind. It reminded me a lot of the Broken Age post-game, where I discovered a lot of new jokes buried behind actions that you'd normally have no cause to take. Maybe that's just something that Double Fine does. If so, I like it.
I actually got all of my broader opinions about this game out in my 15 hour post, so I have nothing new and profound to wrap up with. I liked the game a lot. I will probably play the sequel if it doesn't evaporate into vaporware. And I see no need to ever revisit it after this.