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.