I think a book is very classic. It can be literature, or something science related.
If you want to buy tickets (to a game, concert, opera, etc.) it would be good to get in contact with his wife and see what he likes. There's no point in getting him tickets to something he hates.
Also, I don't know what they cost, but there are likely tickets to things like balloon rides, sky diving, and such. If you think he might enjoy something like that, consider that as well.