http://www.realgap.co.uk/Home and
http://www.workandvolunteer.com/ may give some inspiration.
Lots of similar sites looking at one week to one year placements and lots of ideas offered, most which require either a fee or offer suggestions to get sponsorship for the tasks / charities involved. if its a charity only venture you fancy - choose the one closest to your situation, contact them, and see what projects they have in okace. If its something like an animal thingy - what about volunteering at your local dog or cat rescue?.........
Not sure of a 'challenge only 'site, but there may be something if you did a search on 'personal development' perhaps too.
I would say decide on what you want to achieve - stay in the comfort zone doing what you are familiar with or take a leap - the bungee for example as suggested.....? Go away and take in sights and give to another culture or volunteer teee planting in the Scottish Borders?
Whatever you do have fun at it.!!