I found some gym rings in a park the other day and tried an iron cross and some pull ups and stuff, then I tried to do a dip from the starting position of the iron cross. IT WAS SO DAMN HARD!
First of all, just to clarify I did not achieve full extension on the iron cross, I got about half way. I mention this because someone that could do the full iron cross might scoff at the dips.
But the dips I must say are pretty sweet to try, I was only able to do one and normally I use no less than a 45 lb plate around my waist when doing normal dip sets at the gym.