Any game that's grid based with a fixed number of states works well, either turn-based or real time.
You can do a double elimination bracket like a sports tournament, and can be done on a spreadsheet or whiteboard.
For more than two players at once, you could try snake, hearts, or poker.
A 2 player game, the court is divided into 12 spaces (6 either side of the net). The serving player rolls two dice and moves the ball that many spaces. So 12 is out, anything below 6 hits the net.
Following a successful serve the opposing player gets to choose whether to roll one or two dice. Other than that its normal tennis rules. It's mostly luck with the main skill deciding whether to roll one or two dice.
Or how about Rock, Scissors and Paper?
https://en.wikipedia.org/wiki/AI_Challenge http://ants.aichallenge.org/
Despite the name I don't remember ML featuring prominently in the solutions.
Can quickly spin up a Snake using an AWS lambda. It can be a lot of fun.