There is plenty of stuff for guitarists to spend their money on. In fact, I wouldn't be surprised if guitar players were the most marketed audience among musicians.
Maybe you know someone that just picked up the guitar, has shown a lot of potential and commitment, and you'd like to feed the dream a little with a well-targeted gift for the holidays. Here are some ideas to feed the dream.
Pedals
Pedals are basically little electronic boxes that you can plug in between your guitar and amp and can give you different sounds like distortion, overdrive, echo or delay, reverb, modulation effects, and so on and so forth. Have they been learning Enter Sandman but their amplifier doesn't quite deliver the metal badassery that the song calls for? You probably need a distortion pedal.
A description of the popular sounds is a little beyond the scope of this article, but there are two types of pedals: a dedicated pedal and a multi-effects pedal.
A dedicated pedal will do one sound very well and offer a little variation on that one sound. A multi effects pedal will offer many sounds but might not deliver them as perfectly as a dedicated pedal. A dedicated pedal might cost anywhere between $30-300 for one or two good sounds whereas a multi-effects pedal might cost $100-500 for 10 or 20 good sounds and a lot more flexibility.
For someone just beginning to get their toes wet, I would recommend a multi-effects pedal. Then once they find the sounds they use the most, they can collect the necessary dedicated pedals.
Boss Metal Zone (popular distortion)
Boss Tremolo (popular modulation effect)
Digitech RP55 Multi Effects Pedal (entry level)
Line 6 M9 Multi Effects Pedal (more advanced)
Capo
A pedal wouldn't be too practical for someone that only has an acoustic guitar, but a capo would be practical for both acoustic and electric. A capo is basically a metal bar with a pad on it that acts like an extra finger so that a player can move their open chords to other keys.
Capos can be very useful when you want to change the key of a song but not learn any new chord shapes. I'd say this is a must-have for players who learn songs to sing and play them, rather than just straight ahead guitar playing and no singing.
Shubb Guitar Capo-- the one I recommend most
Magazines
Some of my favorite guitar publications are Premier Guitar, Guitar World, Guitar Player, and Guitar Aficionado. It's a great way to broaden one's exposure to new music, products, trends, artist interviews, gear discussion and analysis, and music lessons.
Picks And Strings
A guitar player can never have too many picks or strings. Sometimes though, these things come with very personal player preferences. Make sure you check in or do some detective work before you spend money.