Here are some ideas:
- Stroll down the main streets of St. Helena and Calistoga. In St. Helena that's Main Street which is also Hwy 128. St. Helena is more high end, in terms of having some really uppity shoe stores and design shops, but it also has fun and funky boutiques. Calistoga is more of a "western" town and it's main street runs East-West and is Lincoln Ave. Stuff is cheaper there.
- Visit a spa in Calistoga, especially one of the ones with natural hot springs where you can take a mud bath or regular bath or get massages. I like the Mt. View hotel & spa in downtown Calistoga. It's a volcanic area and a lot of the spas tap into natural hot springs (which stink, of course, but are supposed to be good for you)
- Drive down the Silverado Trail -- it's the road on the eastern side of the valley. Very beautiful drive, especially the section from Napa town to Calistoga
- For eating, in St. Helena two very good restaurants are Cook and Cindy's Backstreet Kitchen. Cook will always make something vegan, while Cindy's has some vegetarian and vegan things on the menu. Farmstead is also reliable, just south of the main part of town, and is a bit cheaper. In Calistoga, there's good cheap mexican food (Pacifico) and fancier fare at Brannon's and a couple other places. Or, stop at Dean & Delucca just south of St. Helena, pack up a picnic, and go eat at V. Sattui winery, they have a lovely picnic area with trees and lawn
- Take a hike in Bothe State Park, which is between St. Helena and Calistoga on Hwy 128. Do a little loop, it's easy enough and very pleasant in the shade on a hot day.
- Tour the winery at Castelo d'Amoroso. The nutty owner built a replica of an old Italian castle, spent over $30mm on it. It's a trip even if you don't drink wine. Or go to Sterling and take in views from up high looking over the valley.
- There are some places to taste olive oil; the only one I know is at Long Meadow Ranch, which is run by a friend of mine. It's a drive up in the hills west of St. Helena, but a beautiful spot.