Possibly the worlds greatest odd sporting event, the recent Cheeserolling championships at Coopers Hill, highlighted the fun that can be had with just an extremely steep grassy slope, fearlessness of being hurt and a nice mature cheddar.
But if you have the first two and are lacking the cheese you could do worse than play Downhill Giant Slalom Running. This invented sport was featured on Channel 5's International King of Sports in 2004. Basically it's a running version of skiing's giant slalom and all you need to play is as steep a slope as possible and a set of poles to makes the slalom gates. Then, simply, take it in turns to run as fast as possible down the hill, passing through each gate in turn. The key to a fast time, just like in skiing, is to stay as close as possible to the inside pole. The fastest down the course wins.