Markleeville Death Ride, in the Sierra mountains of California. 3500 riders, 14,500' of climbing, 124 miles. Just a ride although there's some fast riders.
Everest Challenge: 29,035' of climbing in two days. It's mostly a race but there's a "tourist" division too.
Mt Evans hillclimb, from 7000' to 14,000'. There is a race and a public category age graded race/ride too.

I've done all of those. They're all fun. The Death Ride is the largest event of the three.