Any genres in mind? Shattered Pixel Dungeon is a pretty fun one, IMO. I like Idle Fantasy for its vaguely RuneScape feel (though it's largely text-based, so don't expect much graphics) while letting you queue actions up for hours of letting the game run idly. 2048 is classic. Luanti (similar to Minecraft) is available on mobile but such a huge battery drain I haven't played it much TBH. And UnCiv will let you play a game similar to Civilization. I have all those on e/OS (de-Googled Android fork) and as I recall they are all open-source and ad free.