A few ideas for games in the library:

Scavenger hunt - I agree with Sue that this is always good
Trivia contest
Find the ____ (picture or thing) - we do this with kids all the time but would work for anyone
Name that cover (close up picture of a book cover)
Name that book (from a description)
Who wrote that (ID a quotation or book's author)

Have them read different genres to complete a bingo-type card ... westerns, mysteries, sci fi. Once they bingo they get a prize. You can make it short - three across, three down so they only have to read three books to "win".