If you have five spare minutes, Wheels Plus Pairs will fill them. Everything happens on one screen with a 6×6 grid with 18 pairs, and the whole thing runs straight in the browser - no downloads, no sign-ups, no fuss.
Two cards flip per move; match them and they stay open, miss and they flip back after a beat. The whole skill is building a mental map of what you have seen without losing your place. With a 6×6 grid with 18 pairs, later flips are genuinely tense - you know the pair is out there somewhere.
It suits anyone with a queue to wait in or a kettle to watch: students between classes, commuters, parents stealing ten quiet minutes. There is no meta-progression to lose and nothing to catch up on - a week away costs you nothing but a little sharpness.