After I collect 20 checkpoints, there is a checkpoint on the roof of a building across the street from that little underground minimall thing, anyone know how to get it?
Take a ride around there before you do the playground. Behind that building is a white complx with kind of a ramp that leads up to a closed doorway with a body armor. But, if you go down the small alleyway leading up to the ramp with a motorcycle, you can use it to jump to the next building. It's also a Unique Jump. I'm not sure you go the full Unique Jump distance to get to the checkpoints, though.
It's best to take two runs at it, first getting the shorter one, then turn around, use the ally, and jump the distance to get the last checkpoint. Make sure you're not going at FULL speed, but somewhere near there when you jump.
P.S. Make sure it's not raining, otherwise it's too hard...