For short bursts, nothing beats Super Hexagon. If you're clumsy like me, games won't last 60 seconds. :) Great chiptune soundtrack as well.

The Incredible Machine ones are good. Boot up, hit a few inventions, leave.

Believe it or not - Dark Souls can work very well in 15 - 20 minute sessions. I spent about 30 hours on it like that - just levelling up my character, exploring or making multiple boss attempts without getting too aggrevated by them. Actually made those encounters a bit easier (for me) too.

Steam Marines is another good one - a mix of squad tactics, randomised levels and permadeath. Quite short to completion (if you make it). Hopefully we'll see it on GOG when finished.

Chainsaw Warrior - based on a single player board game by Games Workshop from the 1980s, you are tasked with saving the world from some b-movie evil in an hour. Not easy. Good fun once you get going and can take between 15 minute (if you fuck up) and a couple of hours max to finish a game.

...just stuff I play off and on really, but might help give you some ideas!

Borderlands. Get in. Frag a bit. Get out. Anyway the whole save-spawn system makes the game discontinuous by nature.

Spacechems. One factory per day.

Scribblenauts. Type a couple of words. Exit before getting eaten.

Kingdom Rush. Why hadn't you passed this level yet ? Ah yes, remember now. Bye.

Europa Universalis 3. Launch. Scream. Turn computer off. Play cup-and-ball.

Driftmoon works well. Each region is small and easy to explore. Quests are short and inventory management is basic. A good little RPG that can be played in short bursts. Also allows you to save any time.

Huh. That's good to hear. Have to figure out which versions of the patches work with current game version again, but I am really interested to pick this up again.
That's good to hear as well. Haven't started it yet, but am awfully curious. Looks good!

Driftmoon is pretty cool, you will enjoy it a lot :)

If you are looking for something quick to get into on breaks or if you don't have enough time and you fancy RPGs and a challenge, look up some "coffee break roguelike" games. Quick and easy to get into. They also have a few on mobile.

If you have time, I do recommend for anyone to search for and watch this video for a few broader ideas in the roguelike genre:

Google: Top 15 Roguelikes for Beginners and Experts Alike