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A safe bet for most people would be either a $10 Google Play or iTunes gift card. Except that Apple made it $15 minimum for the iTunes gift card, so you better like those people 50% more.
Another thing that you might consider are those little iLove pop up speakers. I just bought a couple of the iHM60GT speakers for around $17 each. They plug into any 3.5mm jack, so they will work with almost anything and sound pretty good for a tiny portable speaker. The X-Mini 2 is similar and priced about the same if you want a different style.
If there is a Linux fan you are shopping for, you might consider Rebel Code: Linux and The Open Source Revolution. It's a pretty good book, and you can pick up a paperback edition for $10-15.
If you want to spend a little extra, I personally would suggest an Official Red Ryder Carbine-Action Two-Hundred-Shot Range Model Air Rifle. It's useful against bandits and bears.