What are the best games for 15-30 minute play sessions?

Coming from a summer break where I could basically play games all day long, I've been thrust into my first year of medical school. I'm finding that it's significantly harder to dedicate significant periods of time to gaming, so I'm looking for games that I can play in some of the breaks I have throughout the day.

For reference I have access to a PC, PS4, Wii U, 3DS, and Vita, but if there are any games that aren't on those consoles that you'd like to mention, by all means post away.
Picross, Tetris, Doctor Mario and all kinds of puzzle games.

Also: Breath of the Wild.
Play the first hour. And then go for the shrines. Discovering and completing them will make for more than a hundred of satisfying mini challenges that can be completed shortly. And once you've beaten at least half of them, finishing the main quest is fairly easy.
Any short multiplayer games or get into mobile gaming.

Single player games like Zelda wont work cause you won't really be able to fully enjoy and appreciate single player games of that magnitude having such short game times
My schedule had become packed, and as a result I would only play games once or twice every few months. I really though that my days of gaming were over. Then I got a switch and that changed everything. I can easily squeeze 5-15 minute sessions in throughout the day. It's sleep function works really well, so that any game can be played in small bursts.

I guess what I'm saying is: get a Switch. It's totally worth it if time is your biggest issue.
Destiny. You can run a couple strikes, play a couple Crucible matches, or run some dailies. It's quitting after the 30 minutes is the problem. The game always wants me to keep going.
Do you like multiplayer games? My go to for short sessions tends to be fighting games. Just play a few matches with the AI or online. My current fighting game of choice is Street Fighter V, but any fighting game will do really. PS4/PC have access to all of the relevant ones.
Sonic Mania is my vote.

The game gets straight into the action, and accomodates making good progress in short time while being easy to find a stopping point. It also has an incredible amount of replay value with other characters, hidden secrets, and bonus modes like Time Attack and
Mean Bean Machine.

jon bones

hot hot hanuman-on-man action
Only one answer, Rocket League
If it clicks, it will be perfect.

Deep enough to be a dedicated "go to" game, but with simple mechanics so you don't need to use any precious brainspace on it (unlike stuff like R6: Siege)

Congrats on getting into medical school, good luck on the long road ahead.
Single player games like Zelda wont work cause you won't really be able to fully enjoy and appreciate single player games of that magnitude having such short game times
Subjective I know, but Zelda was the perfect 15-30 min at a time game for me. The flexible save system was a big contributor I think. Made it easy to turn on for a few minutes and get to that vista or that next shrine I wanted to get to, then save and come back to it later.
Resident Evil: Revelations is a single-player game specifically broken up into separate 20-minute chapters, and every few chapters there's a "Previously On Resident Evil" montage that gets you up to speed on what happened earlier in the plot.
Just about any rhythm game. PC stuff like Stepmania and Lunatic Rave is particularly great if you're doing something on your PC before and after the 15-30 minutes.

This game would be perfect if you could skip the Pearl/Marina bits. I love them but it wastes at least like a minute every time I start up the game.
If you get access to a Switch down the road, Breath of the Wild is pretty good for this. It feels like a lot of the shrines in the game were designed with portability/short play sessions in mind, as there are typically puzzles that can be solved in 15-30 minutes.

Also PUBG is pretty good for this as well, provided you can limit yourself to 1-2 matches a day.
Pac-Man 256
Pac-Man Championship Edition 2

Both on Steam (and PS4?). Love these. Pac-Man 256 is surprisingly addicting. You can play it in short bursts, but it's so hard to stop. It has that "okay, just one more game" quality.