I have a soft spot for these kind of "meaningless" challenges. Personally I'm now trying to figure out how to ride a 100K, but using the least amount of road to do it. So basically riding 3-4 hours in a roundabout is the best. Challenge is finding something with the appropriate radius. Too small and it's hard to keep up the speed.
Another one I'm doing is using wandrer.earth to track the amount of unique roads I've traveled (so opposite of 100k challenge). Trying to cover every street in my city.