The concerto for two pianos is great fun, but let's not totally forget his 'single piano' concerto. It might be a little less 'wild', but it's full of beautiful and fun tunes, and has that same almost fragmented, 'you-never-know-what's-going-to-happen' joking around (listen for instance to the crazy ending of the first movement and most of the last movement)
I also like a lot of his delightfully unpretencious solo piano works, like the Suite pour piano and Suite Francais (also scored for what I believe is wind band and harpsichord) Some of his stuff is a little too easy and kitschy even for me, though, and sometimes I can get a little tired of certain clichés of his, things that just sound 'too Poulenc', once you get to know him a little.. But then there's the other side of his famous Janus-face; the church music. Apart from the Gloria, Stabat Mater and Salve Regina, I know he has motets, a mass etc that might be worth checking out..