Ok I have a guitar weilding friend I don't see much and we always buy each other something really small but excellently thought out, so its more a gesutre of 'I thought you'd love this...' rather than make each other akward with the money spent. Last year for his birthday I bought him something which made his face light up and make him ten times more annoying.
PIKCARD.
It's exactly like a credit card only it has various designs on it from swirls to skulls to animals and has four pik size punch outs. The idea being it stays in your wallet till that random momment when you find a guitar - party, shop etc, where you can whip it out, and look cool.
type it into a search engine and it will throw up lots of sites, or you can get them from ebay. And at 99p each its the perfect, I knew you'd love this, but it didn't cost too much.