Road biking on the summit is spectacular and can be as challenging as you wish to make it. It is, however, limited to the single paved road that runs through the summit, Donner Pass Rd which is Old Highway 40. The entire route is shown in the map below with the elevation profiles. It can be done in its entirety from either direction or it can be done in parts starting anywhere on the route. The most challenging, and spectacular, section of the ride is the Donner pass ascent. The ascent is 3.1 miles with an average slope of 7% and maximum slope of 16%. The climb is open all the way with great views of Donner Lake and Pass to distract you from the burning in your legs. The ride across the historic Rainbow Bridge is a unique riding experience. Just make sure your brake pads are in good condition if you are riding back down the pass.
As a comment on the quality of the ride through the Donner Summit area, the Amgen Tour of California is including it in their 2011 ride.
When I was in my twenties I used to live in a cabin on Donner Lake, I worked at the North Star Ski resort for a time and at O B’s Board in the town of Truckee. I bought a bike at a shop in west gate and used it as a primary form of transportation back and fourth between Squaw Valley, the Truckee Airport and where I lived at Donner Lake. I rode up the summit a couple of times just for something to do and remember how difficult it was by the fact that I remember hitting first gear for a while on a ten speed bike. I am fifty two now and will try it again some day soon, hope to see you there, it is a must have for your to do list. Can’t beat the view on the way down, don’t expect to ride wide open going down, check your speed with your brakes often. You will have a feeling of zero accomplishment if you have some one drop you off at the top and coast down, so don’t puss out. I also recomend the Pacific Crest Trail between Donner Summit and Squaw Valley, you can make it one day if you start early bikes were not allowed when I hiked it.