Zoo Boom

Tap two or more touching animals of the same color and they pop, which is the entire input this game asks of you. What keeps it from getting stale is the light task layer on top — clear a certain number of one animal, or hit some other small goal — plus the fact that a bigger group popping together triggers a visibly bigger payoff than a minimum match does. It's calm rather than competitive: there's no timer pushing you, so a level that looks impossible at first glance is usually just asking you to look at a different corner of the board. The difficulty creeps up slowly across its levels rather than spiking anywhere obvious.