Bring the tandem to Yosemite Valley on a weekday. 10 miles of bike paths, can't get any better scenery. Great way to see the valley.

Otherwise, Napa Valley is a decent choice. Start in Yountville, cut across to Silverado Highway, head up to Calistoga and back. Great food choices in Calistoga and Yountville, nice scenery, wide bike lane on Silverado Trail, mostly flat. Mumms is a great place to stop on the way back.