If you live around here and like to drink wine, I hope you've made it up to the wine country by now. If you're looking for a non physical activity with nice driving (or bicycling), this is one of the best options out there.
Required people - 1+. How much fun is it alone, though? And for the winneries that either 1) restrict you to X tastings out of Y choices or 2) pour 3 inches per taste, it's nice to share. This is a very good date, either with someone close, or someone you're just meeting. If you go in a larger group, you can have a couple drivers so that you don't need to worry as much about getting drunk. Note that the winneries tend to be on winding country roads - not an appropriate place to be driving near your limit.
Required gear - none. They'll provide the glasses. A notebook can be useful to record your evaluations, as well any interesting information from the staff. Don't rely on your memory - if you visit 3 or 4 places you can easily lose track of which wines tasted how. Alternatively, you can write notes on winnery literature, but that's a bit more scattered when you look for it later.
Required time commitment - half day for local winneries, full day for Napa or Sonoma counties unless you're already near there.
Costs - variable. In Napa, you will almost certainly be charged for tastings. This was implented years ago in part due to the tendency of people to abuse freebies and drive drunk afterwards. But it's also true that the tasting room can become a profit center as a large portion of visitors will never be back again. That could mean $5 for 4 tastings, or $2 each, or in a few cases it more closely ressembles a wine bar. Sonoma is quite different - very few places charge, or it is a token $2 for up to a dozen different tastes at Rabbit Ridge. Sonoma winneries tend to be smaller and in my opinion, far friendlier to visitors. Less rudeness, far less traffic. It's a no brainer to me, especially since I like Zin and Pinot more than I like Cabernet. In between the two is Carneros - some costs, more politeness, and the best Pinot around. It's also the closest from Vallejo if you're running late.