Amazon Prime is life changing, especially if you have small children. Buying someone a year of prime will make them happy every time they use it (and they'll think of you). Sure you get free two-day shipping on just about everything, but you also get free returns, free streaming video, free streaming music, free kindle books, free online photo backups, and more.
The only negative to giving this gift is that it comes in the form of a "Gift Email". There is a chance that will get spammed or glossed over so you'll want to send them a heads up first. If the person already has prime, they can redeem your give to add a year onto their subscription anyway so you're all good.