The new-ish wine museum up in Napa is actually quite fun. It's not the exploratorium or anything, but the rather modern building houses some interesting exhibits on winemaking, food, a winetasting bar -- even stuff froom Julia Child's Kitchen. By far though, the best part is the sniffable Edible Grden, which is planted with various herbs and fruits and flavors that you find in wines. They've conveniently separated out the gardens into zones, so you can find things that go with big reds like cabernets, or whites like chardonnay.
If you're already up in Napa and tooling around the wineries and shopping outlets it's definitely a nice place for a break.