I always bring henna on river trips. If you live in a town like Jackson, Boulder, Flagstaff, Moab, etc. where there are a lot of hippies, you should be able to find some at a hippie store; if not, you can order it from Earth Henna.
This was also a big hit: disposable plastic shot glasses duck taped to an oar for a giant ten-person shotski (or shotoar):
And a second on the blacklight:
Games? Who needs 'em!? Just spend hours of your night trying to put your thumb through the bottom of an unopened beer can until you master it and become so drunk that your buddies make you wear a life jacket and helmet at Ledges camp.
Oh, and I havn't heard any mention of bocce yet... but I think that's a required item for any river trip. Make sure you get the kind that glows in the dark so the fun doesn't end when you lose all your balls in the dark.