Sonoma County from Oakland, Transit-Assisted
With a bit of a train assist, it's possible to be casually riding through wine / cider / beer country in central Sonoma County (roughly within the Santa Rosa - Guerneville - Healdsburg triangle), on the Joe Rodota Trail and West County Regional Trail:
In my stronger days, I'd biked to Sonoma County all the way from SF or Oakland in a day (~70-100 miles one-way); I wrote up some of these rides as SF -> Russian River Brewing (Santa Rosa), SF->Petaluma-and-back, SF->Healdsburg overnight, and so on. But I'm not really interested in (or capable of?) a ride that long these days-- 40-50 miles is my current sweet spot for an extended social ride with plenty of stops and exploration.
More recently, I've taken four trips with a similar blueprint: bike or take transit to the San Rafael Transit Center, take the SMART train to Santa Rosa, and bike 20-40 scenic miles from there, using multi-use paths where possible. I'll stop in Sebastapol or Forestville for lunch, stop later in Healdsburg or Windsor for a snack or beverage, then get back on the SMART train to head home.
Rather than write up every variant on this in detail (especially since SMART stations and track-adjacent multi-use paths are still changing year-to-year), I've put together a sort of choose-your-own-adventure map, merging variants on this trip I've done:
The above would be ~180 miles of riding, which I'd never do all in one trip. But here are a few interesting subsets of this route.
Doubly Transit-Assisted: (27-38mi):
For the shortest, most transit-assisted Sonoma County trails ride:
Take GGT bus 580 from El Cerrito del Norte BART (there are similar buses from SF) to the San Rafael Transit Center. Note that this bus runs once/hour and only has room for three bikes on a front rack, so this isn't really an option for a group.
Take the SMART train from San Rafael to Santa Rosa (comfortable, scenic, plenty of room for bikes)
Do this pleasant ~27 mile ride (or a ~38mile version if you include Healdsburg), ending at the Windsor or STS SMART stations. There are a lot of good food options in Sebastapol, and wine and beer options in Windsor at the end of the ride
Take the SMART train back to San Rafael
Take the GGT bus back to El Cerrito del Norte BART
I wrote up a recap of doing this with a friend in "SMART-assisted Sonoma Trails", though note that the Windsor station was not yet open so the last few miles to STS airport station are no longer necessary.
I've only biked this variant once-- while there's novelty in getting to Santa Rosa entirely on transit, to me it's not quite worth the time and logistics of connecting multiple forms of low-frequency transit in both directions, as this adds time and more need to stick to a specific schedule. Instead, consider:
Three-Piece Ride (70mi split into 20/30/20):
This is probably the most straightforward and group-friendly variant, I've done it a few times.
Bike to San Rafael (whether from Oakland/Berkeley on the Richmond-San Rafael Bridge, or from SF over the Golden Gate Bridge), about 20 miles
Take the SMART train to Santa Rosa
Ride the above 27 or 38 mile Sonoma trails loop, ending at the end of the SMART line in Windsor
SMART train back to San Rafael
Ride back to Oakland/Berkeley/SF (20mi)
While this is 70-80 miles of riding, it feels easier than that, because it's broken into three rides with an hour of relaxing downtime on the train between each one. I usually plan to get to San Rafael at least 30min early on the way up, since the SMART train runs infrequently-- missing it due to a roadside flat or extra photo stops would be disappointing. That means we usually have buffer time in San Rafael to get food (Sol Food?), or at least some to-go pastries to eat on the train, and if we're running later we just get lunch on the other end.
This split ride also leaves open a last-minute option to skip the final 20-mile segment and catch a bus from San Rafael, if I'm riding solo or with just 1-2 friends (as noted, the 580 bus can only take three bikes). Even if you don't do this, knowing this safety net exists even for one person in a group if they are feeling exhausted when we get back to San Rafael can make the day feel less daunting. There's also a similar (and even more occasional) Larkspur -> SF ferry option if you're early enough in the day.
SMART Pathway San Rafael -> Santa Rosa? (+45mi)
As a variant on the above, you can (mostly) ride between San Rafael and Santa Rosa on the "SMART Pathway": multi-use trails paralleling the train tracks connected by some road segments. Parts of the route are somewhat boring views of the tracks or highway, but I still found it pleasant and low-stress:
I even fit in a few weird dirt path/park detours or shortcuts, like this trail by the Beverly Ehreth Ecological Preserve slightly outside Novato, to cut between two road segments...
I wouldn't personally add all of this SMART Pathway riding to the already-long 70-mile day above, but I've done a variant on the overall route where I stay overnight in the Santa Rosa area, making it less daunting to do a full 60-70 mile ride between Berkeley and Santa Rosa one way, on the day I'm not also riding around Sonoma county.
One benefit of riding generally parallel to the train is you could decide partway along this segment to hop on the train at an intermediate station.
+Dirt and Wetlands?
On my most recent ride, I got tired of riding along the train/highway South of Novato, and diverted out onto some dirt trails (at times rough or narrow) I saw on the map, along the Hamilton Wetlands(?), with many birds to observe.
Just look at that environment! I'd consider riding here even just as a San Rafael area ride, though this was a weekday-- I imagine it might be even slower on a weekend if more walkers are out on the trails.
+Scenic Marin / Nicasio?
Another possible variant between Petaluma and San Rafael includes Nicasio and Lucas Valley Road-- a popular Marin road riding segment. This is beautiful, swoopy, and has multiple cheese shops to stop at-- though it also has one relatively narrow road section West from Petaluma that I don't love when traffic is heavy.
Any version of this ride easily becomes an all-day adventure when you include the transit-- I don't recommend it if you're on a strict schedule, but I enjoy being able to stretch the boundaries of where I bike in the Bay Area, without having to drive bikes to the start of a ride.
For a different spin on riding to/from Sonoma, see also "3 days of bike camping, Guerneville->SF", a pleasant 45 miles/day trip with a few friends.