They should do one tour, I find that is enough for most people. Benziger, in Glen Ellen, has the very best! Includes a tram tour that takes you to the vineyard and explains the wine making process and how the grapes are harvested. It's also a beautiful venue and the wines are divine.
CAUTION: They retail wines at local markets and the wine is less expensive at Safeway, etc... Only buy wines there that are NOT for sale elsewhere, otherwise they will over pay.
Oftentimes tourists believe that wine is priced better at the wineries, not usually the case if they distribute to grocery stores. Ask the wine educators before purchasing, they will be honest with you.
Viansa is a great stop too - 90 acres of wetlands preserve, magnificent view, Italian varietal wines, live music on the weekends and one of the few wineries that serves food. Visit around lunch time for a bite to eat. Full deli, outdoor grill, wood burning pizza oven. The wine at Viansa can only be purchased at the winery, no distribution.
Gloria Ferrer is across from Viansa and has sparkling wine - the Blanc de Noir is my favorite! Get a glass and enjoy the view from there as well. Blanc de Noir is distributed to local retailers, but the winery sells it for about the same price that you can find it at the market ($18.00 a bottle).
Both charge about $25.00 for tasting but worth it!
There are soooo many fantastic wineries and each one is unique and wonderful in it's own way. As they drive through the wine counrty they should stop at anyplace that inspires them.
Be sure to bring lots of water to stay hydrated, eat, and don't feel obligated to drink every taste that's poured in your wine glass. It's perfectly acceptable to have a taste (usually you need two to truly appreciate the wine) but after that --use the dump bucket unless you absolutely LOVE the wine. Otherwise you'll never make it through the day!
Don't miss the Sonoma Plaza - excellent restaurants & fun boutiques. Great walking trails, General Vallejo's House, Mission Solano.
Check with the Sonoma Visitors Bureau for a calendar of events the weekend they plan to visit. Always something happening on the Plaza on the weekend, and Tuesday night Farmer's Market is not to be missed.
There are great tour companies here that can provide transportation.
If they are staying overnight I suggest the Best Western on 2nd Street West in Sonoma, walking distance to the Plaza.