Here's the famous Haleakala Crater. It is huge: 6.99 miles (11.25 km) across, 2.0 mi (3.2 km) wide, and about 2,600 ft (790 m) deep. The interior of the crater is dotted by numerous volcanic features, including large cinder cones. Gorgeous!
We saw lots of bicyclists on the road. Many people ride bikes all the way up and all the way back down the steep, winding road. I have to say I would never want to do it--I'd be scared out of my mind. It has to be unbearably hard work biking up, not to mention trying to stay out of the way of cars. On the way down, I can just imagine losing control and falling in front of a vehicle. Yes, I usually think of the worst possible scenario. I was scared just riding a bike to the Golden Gate Bridge and going across in the bike lane. :)
It's also common practice for people to arrive before sunrise to watch the sun come up, or to come out in the evening to see the sunset. We did neither and were there in the middle of the day, but the crater was still hugely impressive.