K.P.
Beer? Seriously (not meant to offend anyone), but college kids celebrating a 21st birthday typically want some booze. Nothing "off the wall", but snacks and a pinata simply aren't going to do it here.
Definitely pick a theme and go 100% with it. Luaus are fun and easy to do. College kids LOVE wearing costumes, so if you could incorporate that, definitely go for it. Go with a 90's sitcom theme and see what that brings! Think of the possibilities... Seinfeld, Frasier, Full House, Roseanne, Friends, Married with Children, Fresh Prince... so many ways to participate for everyone!!
Have fun and remember that while the grandparents are invited, the party is about the twins!