Given overtaking is impossible for similarly matched cars and drivers can avoid pitting simply by driving slowly, what kind of event would you have for Sunday?

Best I can think of is an elimination style event: cars are released from the pits in reverse grid order with a 3s gap between each, and if you can stay within 1s of the car in front for a whole lap, that car is eliminated unless they're also within 1s of the car in front of them at the end of the lap (I guess the last checkpoint would need to be just before the pit entry).

Worst I can think of is a series of duels, where Kevin Magnussen is given an SUV and a 3s gap, and drivers need to successfully overtake in an F1 car within 1 lap.