Observing people interact on the bus, every day as I go to school. One person makes a gesture, another reacts.
Chess, ping-pong, sitting quietly in a park and watching the wildlife, go to the lakeshore and listen to the water against the shore.
Looking at visual arts, both ones that appeal to you and others that disgust you.
Film, go look at some independent films, stuff that was made early in film's history. Salvador Dali's "Un Chien Andalou" can teach you a lot.
Anything and everything to do with computers, web page design, Schockwave creation, programming emotion within MIDI files, new media art, diagnosing computer problems.
Studying other instruments, especially how they make themselves dissimilar from the piano. What other instrumentalists do, that pianists do not.