A great way to work on core and upper body strength, as well as bilateral hand use is to put your climbing rope in a lycra swing. Its really hard to get any traction/stability from your feet and you get to rely on upper body and core strength to get yourself up high enough to rescue the monkey!
The kids find it to be hard work and are often exhausted after doing this activity but they get a great workout with it.
What are some variations that you can think of?