Complete objective with the LEAST amount of points. IE lock 2 balls, start a mode, start loops on maiden etc . I like the 2/3 locks or something that forces the ball back in the shooter so there’s a good stopping point.
When I did a pingolf tournament based on having to choose a hindrance for each hole, the hindrances were:
Wear oven mitts
Press the flipper buttons only with chopsticks
One handed
Cross handed
Cover the DMD
Cover the flippers
Cover everything but the last couple of inches above the flippers
Play with your chin resting on the lockdown bar
“Drunken lout”: another player stands right next to the game and tries to be as distracting as possible (free to do anything but block your view or touch you or the game… we did a lot of pointing at playfield features and saying “Hey, what’s that? Can you hit that? I want to see what happens!”)
There’s a number of physical challenges that can be suggested, at the risk of seeming ableist, such as pinkies only, thumbs only, sitting down, standing on a platform, cross armed (aka lotus), standing with your back to the table, wearing an eye patch.
Per-game objectives can be easy to suggest and monitor, although they may vary from game to game. Many have been mentioned above. Some are time-based, objective-based, or a combination of both.
Finally, consider “hot potato”-style rotation games, which are not necessarily limited to 4 players, such as Stall Ball (see Stall Ball )