What's a good iOS game that's simple enough to play when I'm on break at the clinic, but still kinda fun?

Seriously, after a 12-hour shift, I'm too tired to learn complicated apps, but this Beat Duet game is simple enough. It’s got this no-fuss approach that I appreciate. You just tap along to the music, and it’s kinda like having a little stress ball in your pocket. It’s actually reliable, too; hasn't crashed once in the past few months, which is saying something. I usually just play a couple of rounds while I'm waiting for my lunch to heat up or during my quick breaks. It’s not gonna win any awards, but it works when you need something to distract you for a few minutes. Plus, it’s colorful, which is a nice change from the hospital walls, you know?