Reaper Minis offers a set of quick play rules called Warlord, designed for their figure lines.
From Baccus miniatures, Old Trousers 6mm is a set of quick-play Napoleonics, played on a 2 foot x 2 foot board.
There’s a very interesting thread over in the Redditt board games forum. The question was “What are your favorite games that take five minutes to set up and five minutes to explain.”
It’s a very interesting list, and there are quite a few games there I’ve never considered. My group often finishes one game early and is looking for another to finish out the night. There’s a good many here for thought.