This was a birthday challenge that I did this year and it was challenging and fun. It might be an intriguing challenge to consider or do 1 x per month. This could be something you test out on your birthday or any other day based upon your age.
The Everesting Challenge is an intriguing idea for those of you who want to do "your own" long endurance event. The history of the idea and rules can all be found at http://www.everesting.cc/the-rules/. The rules are quite simple, find a hill and ride up and down it for as long as it takes to complete 29,029 feet or 8,848 meters of elevation gain.