Our YM planned our combined YM/YW activity this week - "Ice-Blocking"
They bought a bunch of blocks of ice from the grocery store (they got about a dozen of them, which was perfect because we had about 20 kids show up, so they just took turns). Everyone brought a small towel to sit on (dish towel sizes were too big, hand-towel size is better). The concept was simple - put the towel on the block of ice, sit on it, and sled down the hill! It had been raining, so it was slippery already...
It's mid-October, so the kids got pretty cold (sitting on blocks of ice), but it was lots of fun. My daughter thought it was the most fun activity we've done in a long time.
The best part was that the activity was self-perpetuating - they went down the hill, they walked back up, and repeat! They did this for several hours, without a lot of intervention from the leaders. Even though it was dark & cold, they had a blast!