Peggle can be one of the most exciting and frustrating games ever.
EDIT: I find Skyrim pretty relaxing, just going around exploring, listening to the awesome soundtrack, that is until a dragon attacks. Same with Fallout 3, it's relaxing to just explore and listen to the background music, that is until you run across a Super Mutant Behemoth.
Anno 2070, Antichamber, Little Inferno, Minecraft, RimWorld, Space Engineers.
Anno 2070, Minecraft and RimWorld on easiest/peaceful settings so that it'll be more focused on building your stuff without having to worry about hostile stuff (although it becomes more interesting when you have hostiles turned on, admittedly)
These are the ones that I use to relax if I'm not in the mood to play games that
Although I personally can relax in just about any game. Sins Rebellion, Company of Heroes 2, Diablo, Skyrim etc. Perhaps if you can elaborate what kind of games you like to play, I could mention some in that genre I like.