They are adults. Read a book. Go for a walk. Go for a swim. Arrange some security (if that is a problem that concerns them) to have a wander around a local town and explore.
Depends on what they are in to!
I'm going on what the 24/5/6 year olds I play football with are in to. And it does consist of a lot of stuff like that.
Also, being adults didn't stop Brighton from going to paintballing. I'm talking about things purely that will involve everyone working together with a bit of mild competition and banter.