I think katamari is up there with tetris. Great fun but very simplistic. Don't forget to play the sequel. Can't comment on the psp version, although I've read it doesn't really deviate from the standard formula.
Another game that makes me feel nice 'n comfy is rollercoaster tycoon. I prefer the first over the second because in it you can charge for park admission as well as the rides. In the second it is only one or the other.