Where to begin...
I grew up in Grass Valley, so I know that part of the gold country pretty well. Depending upon how much stamina you have, here are a few suggestions.
As you approach Auburn on 49, as you cross the river, go right on old foresthill road, not left as most will do, (great stretch), take that up until it hits Foresthill road, turn right, fast sweeping wonderful stretch of road out to Foresthill. Past Foresthill at the Y, go right on Mosquito Ridge Rd - this is a long loop,the road is super-technical, goes for many miles with NO traffic, has tight and open sections of curves, and loops back into Forest hill road. As you come back on Forest hill road, make a right on sugar pine road, which will take you to Iowa hill and eventually back to I80 - again, mosquito ridge and Sugar pine are tight, technical, curve a minute roads with no traffic, but be careful, as they are unforgiving of mistakes.
At this point you can go north on 80 to Colfax and take 174 to Grass Valley, or go south on 80 to the Meadow Vista exit, go right toward meadow vista on Placer Hills Road - take Placer hills back up to Colfax (fun section) and then take 174, or some miles into Placer hills road, turn right on Dog Bar, which is another tight, challenging road that will take you all the way over to 49 just south of Grass Valley.
From Grass Valley I recommend 49 to Downieville, then on to Sierra City (great road, great scenery if you go early on a sat or sun morning, no traffic), past Sierra city a couple miles turn left at Bassetts and take Gold Lake HWY up and over the hill to 89 - turn around and go back to Bassetts (you'll never drive your elise faster in corners on the street) - again, zero traffic, perfect pavement, great views. At Bassetts again go left and enjoy another great section that leads all the way down to 89 (stop at the valley overlook, if you don't miss it...). Take 89 to 80. From there you have a lot of options (around the lake to 50 and home, or 50 to 89 to 88 at Sornesens,88 to Mormon Emigrant Trail (NF-5), for truly remote motoring (great road in the middle of nowhere that goes back to 50), or back home using 88, etc (cutting back over to 50 at some point is best, as 88 gets a lot of traffic in the valley and isn't as quick a return to the bay area).
If you have questions, feel free to pm me. Make sure you gas up before Mosquito Ridge, etc and have fun.
tim