Too many to list here, but mostly Latin percussion and hand drums, triangle, tambourine, shakers, and cymbals. I'm practicing the vibraphone, xylophone, timpani, and bell parts on an electronic keyboard since there is no room for all of those instruments in our pit.
I've been a drummer and percussionist for over ten years, so I'm pretty comfortable with these parts. On the other hand, I've been playing keyboards for only about two years. I'm not Daniel Barenboim or Lang Lang by any stretch, but I think I can hold my own on simple things if I have enough time to practice.
West Side Story is full of contemporary jazz and Latin music ("Cool," "Mambo," and "America"), while Seussical contains whimsical pop, Latin grooves, folk ballads, and gospel ("Oh The Thinks You Can Think," "Amazing Mayzie," "Solla Sollew," and "Biggest Blame Fool"). In short, both musicals have MANY styles of music that each musician must be prepared to play for long periods of time. It's a lot of work, but totally worth it if you're into this kind of stuff.