There are some good games mentioned already but I'd like to pitch in Planet Crafter. No enemies, no time pressure beyond an oxygen/food/water system, just you and the long-term goal of terraforming a planet by yourself.

The award for almost-but-not-quite-relaxing goes to Hardspace: Shipbreaker, where you can fall into a comfortable trot after you figure things out – and then the storyline makes you want to strangle someone with their own necktie. But the gameplay can be very relaxing.