Rayman Origins. With its beautiful art style and ridiculously catchy music, it's just a feel good game.
Also, someone mentioned Kameo, and i'd agree to the that as well. I feel its a very under-appreciated game, it has a lot of charm to it and it's only huge flaw for me is that it's just a bit too short.
Or if you want a shooter, Serious Sam games are great, as well as Bulletstorm if you don't mind linear design.
As for an RPG, Cthulu Saves the World is endlessly funny, and is a damn decent game to boot.
captcha: graveyard shift
He said fun and happy, captcha, get your shit straight.