It's been several years now since I've bicycled out there, but there used to be an annual ride over Donner Summit. You can take frontage roads and/or Placer Hills road to Colfax and farther east, or at least you used to be able to. I think it might have been the Sacramento bicycle club (Sacramento Wheelmen?) that organized the ride, and they should have route maps someplace. I don't recall Donner as being all that steep, but like I said it's been a few years now since I've been out there.

You could also loop around through Grass Valley and Nevada City and get to the summit that way if you want to avoid more of I-80, but I think I'd use the I-80 frontage roads.

Just googled this link up:
http://www.colfaxarea.com/biking.php

Gives route info over Donner.