There’s lots of fun stupid stuff you can do, at least in 5. Obviously I can’t comment on 6.

• Set a countdown timer and you have to switch right at the timer, then right before the timer goes off drive off a cliff or jump out of a helicopter and the other person has to try and survive. Basically death swap
• Try to steal a tank or fighter jet from a military base and switch after every death
• One person exclusively controls the left half of the controller, one person exclusively controls the right half of the controller, try to complete a mission or a custom objective
• Start a timer and see who can get a five star wanted level the fastest

Basically do stupid shit and don’t take it seriously. Extra points if you both suck at the game.