Did you ever figure out how to do up/downs without really doing up/downs?
I was always trying to figure out a new way to do them.
Generally I'd do hip downs :)
I perfected a way for my knees to hit the deck in a way that made it easier to get up. If you do it right, your chest never touches the deck, and you can spring up quickly.
When I went to bootcamp I found the Marine Corps had their variation of up-downs. The above technique doesn't work to well on hard surfaces.