If you like puzzles, specially hard ones with some nice math and logic try Simon Tatham's Portable Puzzles. I'm currently addicted to sign post, but I can recommend net, light up, and loopy. Depending on the difficulty level, you can spend any kind of time doing them. The puzzles are awesome since you can pick pretty much any difficulty and enter your own parameters. For net I prefer a 25x25 grid, but for signpost I'm currently working on a 7x7.
They're available in android in a really good port. But stay away from the IOS version, you have to pay and it's garbage.