DevilutionX (free, open source, needs gamedata) lets you play Diablo1 on Android, very good time killer (you might need to fetch the gamedata somewhere)
Out There: Omega (paid but one time purchase) is a relaxed starship roguelite
Battle For Wesnoth (free, open source) fantasy style tactical game
Jagged Alliance 2 Stracciatella (free, open source, needs gamedata) - Jagged Alliance 2 on Android, tactical RPG, great timekiller like classic UFO or the old Fallout games.
Notable mentions: WorldOfGoo, Human Resource Machine