Here's one that I still come back to from time to time when I have a few minutes to kill. KAMI is a puzzle game that describes itself as "deceptively simple", and I find that to be a fair way of putting it. Its central mechanic, which, despite the title, is not really about paper, but about color, is easy to pick up as soon as you start playing.

Every puzzle in KAMI is an image formed by squares of different colors. You pick a color and click a square: that square will change color, and so will all squares of the same color connected to it -- like using the bucket tool on MS Paint. The goal is to, in a limited number of bucket-fills, make it so the whole puzzle area has the same color. There are dozens of stages of increasing difficulty, but all revolve around this basic concept.

Aside from the fun and approachable gameplay, the reason I come back to KAMI so much is its soothing aesthetics. Menus have calm music which turns to silence when a puzzle begins, and from there, all you hear is the sound of folding paper as colors spread. It looks great, too: all made using stop-motion with real paper, a trademark of the studio also seen in games like Lume. It's a great game to unwind to, even when later levels start upping the difficulty.