Although the Monterey/Carmel area is lovely and the Monterey Aquarium is fascinating and must be seen, I would save that area for another time.

My suggestion would be to start your vacation with your 2/3 time in San Francisco without the car. On the day you wish to leave the City, take the BART to SFO to pick up your rental car. Drive north back through SFO over the Golden Gate Bridge. Before seeing the Redwoods, check out the Anderson Valley wine region two hours north of San Francisco. You can go directly via Hwy. 101 to Hwy 128 west to the coast. (www.avwines.com has suggested accomodations).

It is a beautiful area, and not nearly as popular/crowded as Sonoma or Napa. The pace is slower and the scenery is stunning and more relaxing (with more of a country/casual feel than Sonoma/Napa). From there, you can drive as far north as you like into Redwood Country (the area around Garberville is really cool and there is great stuff all the way to the Oregon border). If you plan it right, you can return your car directly to SFO the day of departure before your redeye home, minimizing the rental cost. (I imagine that if you picked up a car in the City and returned it at the airport, it would cost more. S.F.-SFO via BART is less than $5 per person).

Good luck and enjoy May in Northern California!