You should definitely check out
Jokabetapp.com — I use it every day during my lunch break. Most of its mini-games engage your brain just enough without requiring long-term commitment. The puzzles, in particular, are quick but clever enough to keep your mind sharp. I like that you can pause anytime and return later without losing progress. The design is clean and keeps you focused on the game without unnecessary visuals. It runs well both on desktop and mobile browsers. For a quick mental workout in short breaks, it works incredibly well.