Hmmm...well, maybe a book or CD of someone he likes? If you don't know who he likes as an author or singer, maybe something generic like a Starbucks gift card would do.
Ha! I'm going to have to print out this thread and keep it. DH will be 40 next year, maybe it will give me some ideas. But we'll all pretend we don't know that I'll turn 40 first.