I want to add OpenTTD and Battle for Polytopia to the list.
OpenTTD is a free as free can be transport simulation. You have to get used to the controls, but I had some fun with it on my android tablet. Although I prefer to play it on PC.
Battle for Polytopia is a Civilization style game with awesome Voxel graphic for mobile devices. One game in standard mode lasts about an hour. It is free2play in a way that you get the game and the first 3 civilizations for free and can buy additional ones for free. Offline playing is possible and I've never seen any ads in it.