I haven't played this, but given the title, you might want to look at Five Minute Dungeon. Tiny Epic * (there are several games in the series) might be good.

I haven't played this, but given the title, you might want to look at Five Minute Dungeon. Tiny Epic * (there are several games in the series) might be good. Although it does not meet your time requirements, it has few pieces and is portable. The random stranger part is tough though. Even simple games take minutes to explain (Hive is an example another user gave, which is a great portable game, but it would take about 5ish minutes to explain the rules). It is also likely that you'll win any competitive games against a new player. Much as I want to promote board games, if you are looking for portable fast games, the app store is probably going to serve you better.