What's a good Android game for killing time on long bus journeys where you don't need Wi-Fi?

Right, so I stumbled upon "Elemental Monsters: Merge" when I was looking for something to play on a ridiculously long bus trip from Hanoi to Sapa. Wi-Fi was non-existent, naturally! This game works a treat because it's mostly offline. You're basically merging these little elemental monsters to make bigger, stronger ones. Sounds simple, yeah? But it gets proper addictive. The art style is cute, and it doesn't eat up your roaming data, which is always a win. I spent hours just merging away, totally oblivious to the bumpy roads. It's not gonna win any awards, but for a free, offline time-killer, it's brilliant. Definitely sorted me out on that journey, and I still play it now and then when I'm stuck somewhere with no signal. Plus, it's easy enough to pick up and put down whenever, which is essential when you're juggling backpacks and dodgy street food!