Starting from any of the major airports around the San Francisco Bay, for example -- or anywhere else near the bay for that matter -- and then riding a long loop around the bay and back to the starting point. That's the basic idea.
(1) Start at SFO and ride south from the airport itself, beginning at the little park right at the edge of the airport, and following the path that runs along the water,
(2) Follow the San Francisco Bay Trail all the way through Foster City and Shoreline Park (near Google HQ) and on as far as the trail goes,
(3) Negotiate the non-trail section as well as possible,
(4) Get on the East Bay trail as soon as it begins,
(5) Hopefully follow it all the way to the ferry at Jack London Square, and take the ferry to San Francisco (which is a very nice ride), and get back on the SF Bay Trail southward to complete the loop... or,
(6) Continue riding past Jack London Square to Berkeley Marina and on to Richmond/San Pablo, and over the Carquinez Bridge to make a much larger loop, ...or,
(7) Instead of taking the Carquinez Bridge, continue through Port Costa to the Benicia Bridge, and
(8) Continue to Vallejo to catch the ferry to San Francisco (which is another fine boat ride), or
(8) Continue following trails and quiter roads as much as possible around San Pablo Bay, through China Camp, and on to any of the ferries from Marin to SF, or
(9) Across the Golden Gate Bridge and back to San Francisco.
There are some additional variations, such as crossing over the Dumbarton Bridge, and continuing around Suisun Bay and the Delta.
I am familiar with much of this, but am trying to fill in some gaps:
--Where exactly can you get back on a trail north of Alviso Marina, in the Fremont/Newark area?
--Can you follow that trail/trail system all the way through San Pablo? If not, where are the gaps?
--How much of Hwy 37 can you avoid by taking trails (I've ridden 37 and find it tolerable but not desirable) or other alternatives close to the water?
--Can you ride down near water level though the Port Costa area? Can you ride along the RR tracks in that section? And is Port Costa still a charming little forgotten town, with accommodations?
--Are there good choices (little or no traffic) around Suisun Bay? For what portions is there a trail system?
--Is there any good camping in addition to China Camp?
--Any other ideas that might contribute to making good tours with these loops?