A gift card is the best choice, IMHO. If he is like most guys, he will appreciate the freedom to choose what he wants the next time he is in the dive shop or book store. Gals may get giddy over the thoughfulness of a useless gift, but most of us guys just cringe and head for the nearest round file to upchuck as the gal always expects reciprocity in kind (some day, I will learn to express my opinions bluntly!)
If the gift card does not work for you, buy or make him dinner.
markm