Monument Valley- I can recommend this game enough. Its gorgeous, its smart, and its as relaxing as you can get with mobile games.
Its so beautiful that you can literally use any of the levels as a screensaver-
It revolves around puzzles that are based on impossible shapes such as the Penrose Triangle. It costs money but it is well worth it. I was so enamored by the game I purchased the dlc's as soon as I had finished it.
Its a brain bender at times but its so peaceful that you feel no real pressure to solve them. You can quietly leave take a break and maybe come back with a new perspective that can crack the puzzle.
Excellently crafted, highly recommended