Holiday Bells has tap-along Christmas songs you can play on your iPad, iPhone or iPod touch. Since there's a full octave of bells, you can play quite a few different songs. These songs are included in the app (click for lyrics):
There are quite a few other popular Christmas songs (such as "Frosty the Snowman" and "Rudolph the Red-Nosed Reindeer") that can be played with Holiday Bells. Since those songs are still under copyright, the lyrics were not included in this version of the app, but try tapping out the songs on the bells and see if you can figure out how to play them!