The Legacy trail starts just out of Truckee at Truckee River Regional park- ~5 miles, paved, follows the river- very pleasant.
The Truckee River Bike trail starts/ends at Tahoe City, also follows the river, quite long, but much of it is right next to the road.
Riding around Donner lake is nice- a little traffic.
All of these are basically flat and very mellow- bring a picnic!