Both SteamWorld Dig and SteamWorld Dig 2 are outstanding games that I've played multiple times and great for when you just want to sit back and relax.
SteamWorld Heist was really fun for a turn-based strategy game (similar to X COM) and I haven't played SteamWorld Quest but I've heard it's a fun RPG/card game. SteamWorld Build is on my wishlist but I have too many other games going on at the moment, though I have heard good things about it. Normally I wouldn't shill for game companies, but this particular series is actually pretty addicting to the point that after I beat the first game, I immediately started a new file and when I beat the second game, I immediately started another file just to get the achievements and then kept playing the game over again to learn new strategies and find new ways to sequence-break the game.
For roughly $5 USD, you can get three games that will easily amount to 50 to 60 hours of entertainment. For reference, according to Steam I sunk 10.5 hours into SWD, 30 hours into SWD2, and 15.5 hours into SWH.