A.S.
My DH always wanted to fly so for his 30th I gave him a ride in T-34 trainer. They're based out of Centennial Airport. http://www.incredible-adventures.com/colorado-sky-fighter... He *still* talks about it 8 years later.
Is there something he's always wanted to do? A safari? There are great deals on Travel Zoo right now, $1000 PP with airfare to go to India for a week, $999 for Vietnam. A huge surprise party?
Key into his interest and it will give you a better idea, or ask your dad, he has some great ideas!