If you're flying into Oakland International Airport (OAK) and driving to Napa, hands down, you should go to Bette's Oceanview Diner in Berkeley for breakfast.  It's so good!  If you're coming in on a weekend, expect an hour long wait.  On a weekday though, less of a wait.  It's one of the most popular breakfast/brunch spots in the area and has actually been featured on several food TV shows.  They're most famous for their delicious pancake souffle (as good and unique as it sounds!).  Don't know how early you're getting in, but they open at 6 AM.

Another reason I suggested Bette's is because the street it's on is lined with a ton of little stores and boutiques.  Tons of unique furniture stores that are great for shopping and browsing.  There's a Crate and Barrel Outlet, Sur La Table, CB2, Restoration Hardware.  Then there's a toy store, an Apple store, MAC, clothing stores, a bunch of little cafes and specialty food shops.  Super quaint and adorable.  Another cool thing there is the East Bay Vivarium.  Seriously the coolest pet store ever.  I go there to buy things for my frog and my friends always love tagging along to look at things.

If you'd like to check out some place in Oakland, I really love Lake Merritt and there are a ton of really great restaurants around it.  You could eat and walk around the lake, or even get takeout and picnic by the lake.  Some good breakfast places around it include Lynn & Lu's Escapade Cafe, Kitchen 388, Grand Lake Kitchen and Grand Tavern.

Albany (California), California is a quaint, lovely tiny little town right next to Berkeley.  Actually, if you walk up the main street Solano Avenue, you'd actually walk into Berkeley.  Solano Ave is filled with cute restaurants and shops.  My two favorite places to have brunch there are  Sunny Side Cafe and Bistro 1491.

If you want some Asian food, you can stop by the Pacific East Mall in Richmond.  You've got Chinese, Taiwanese, Shanghaiganese, Vietnamese, Japanese, dim sum, and the list goes on...  Bunch of cute stores there, too.

After that, as far as walking activities go, things get pretty uneventful.  I can only think of three things that might be worthwhile after that -- Six Flags Discovery Kingdom (though I take it you probably wouldn't want to go to an amusement park), the Jelly Belly Factory where you can see jelly beans being made (pick up a bag of "Belly Flops" -- the misshapened jelly beans that they can't sell regularly that they sell in bulk!), and the Budweiser Brewery, Fairfield, CA if you or any of your friends are into beer.

By the way, all of the places that I mentioned above are both pretty much right off the freeway, so you're not going very far off course from Napa.

Whatever you decide to do, have fun!  Feel free to let me know if you want any other suggestions :)