if it is cars/guns/things like that there is your answer, there is an endless possibility of things you can get. It is always safe, they will like it and use it. Also if you find out what they have been looking to get for a part it works best.
Another thing to consider is something you BOTH will enjoy and get use out of. (Or something you can both enjoy together)
What brought the two of you together in the first place? (Interest wise?)
Or, what is one thing that you have noticed that he needs the most but never seems to have?
If he is going on a long flight to Hawaii and likes to read, 1 of the many e books. I have a sony and use it all the time. Tools and guns are always a hit. My wife gave me a .357 magnum for my 5th anniversary and we are on 27 now.