Steamworld Dig is a fun little game that doesn't take too long to complete and doesn't overstay it's welcome.
You play as a robot with a pickax that goes around digging into the ground searching for treasure. As you dig deeper and deeper into the caves (there are several of them, completed linearly), you get more and more treasure you can use to upgrade your weapons, life, and ability to hold water, which you can use to boost you up over gaps you would not normally be able to jump to.
Eventually you get items which let you jump higher, sprint, and other assorted moves that make getting around easier. All of this is done by digging deeper and deeper with your pickax into deep caves and there are plenty of secrets and goodies to find.
The game adds a small layer of stress by basically putting you on a timer for how long you can stay in the cave with light before the light goes out and it becomes pitch dark, rendering you incapable of doing really anything. It's fun to see how far down you can get, or if you can find a checkpoint in the cave to fast-travel to, before the light runs out. It's a fun addition to the game that never feels cheap.
The game only lasts 5-7 hours, but there is a lot of replay potential if you want to really explore all of the caves and all the game has to offer. It's an enjoyable game that brought a smile to my face and never felt too frustrating or repetitive, and I can't wait to play the sequel.