Skill 5 - 10 Minutes:
Double Unders - Practice your timing, even if you're only able to get 1 at a time. If you are very proficient at Double Unders (you can get 50+ almost every time, without a mistake), work your technique for a set or two, then try Triple Unders for a few minutes.
As Many Rounds As Possible in 20 Minutes:
5 Pull-ups (any style)
10 Pistols (alternating)
30 Second Handstand Hold (facing the wall)
Pull-ups - May scale up to Bar/Ring Muscle Ups (rep count drops to 3 per round)
Pistols - Use whatever scaling you require (plate under heel, etc.). Pistols will be done on the East side of the gym (the side where the barbells are stored). If it an especially large class, there will be exceptions, but this will be where the majority of you will do Pistols.
Handstand Hold - If you can safely Handstand Walk, you may scale up, and do a 20ft Handstand Walk per round. 10ft down & 10ft back... this is why Pistols are being done on the East side of the gym. :-)