Oh, that's a tough one! You want to find something thoughtful and meaningful, but not too extravagant or over the top. I'd suggest something that shows you know him and understand what he likes, like a book on his favorite topic or a personalized gift with a special meaning. If you're still stumped, why not ask him directly? He'll appreciate the thoughtfulness, and you can make sure it's something he really wants. And who knows, maybe you'll get some ideas for future gifts!