A couple games from notdoppler.com that I found particularly engaging, and serve as nice, quick diversions if you’re looking for a break or change in tempo or something:
A bunch of “bloons” float from the beginning to end of the path. Your job? To set up dart-throwing monkeys, cannons, and other towers to try pop all the bloons before they reach the other side. The real nice thing is that the game pauses after each of the 50 levels, so you can play for a minute, then go back to doing other stuff. Although, with apologies to Lay’s potato chips, bet you can’t play just one.
Tower defense of a different sort. You and a computer opponent have a headquarters to defend, and you must train various soldiers and/or build turrets to protect your base while mauling the computer’s. Furthermore, as you fight you gain experience, which lets your military “evolve” to the next age (Stone Age -> Medieval, etc.) A quirk is that you get more experience when your own soldier dies than killing the computers, so you have some more motivation to not just sit on defense.
The computer’s advantages are that it doesn’t actually use money to buy soldiers, and (above normal difficulty) their soldiers are tougher than yours. Your advantage is a special attack usable occasionally which hits enemies over the entire field (EDIT: one age’s special is health regeneration, not an attack).
This game is not pausable, so you need 15-20 minutes to play it through, unless there’s a pause command I didn’t see.
I haven’t invested enough time in either of these games to beat them on their highest difficulty setting. This is probably a good thing.