Book Yosemite now for next summer. You can still find good availability for 2009 if you call exactly one year in advance, not one day later. Many dates sell out one year in advance. There are no timeshares near to Yosemite Valley, and there is too much to do at Yosemite to spend all day commuting to and from the park anyway.
Stay in the park and you are guranteed that you can visit the park every day.
The park has a limit for the number of persons allowed per day, and when they reach their maximum number of visitors they turn everyone away. If you got a late start and had to drive to the park, you might not get in unless you had a reservation.
I would not stay less than 4 nights/5 days IN Yosemite, and at least 3 nights/4 days in San Fransisco. Factoring in 1/2 day or more driving time from SF to Yosemite, and another 1/2 day plus back to SF, and you have filled up a weeks vacation. We stayed in tent cabins one time, and we stayed in Yosemite Village the other time. I liked Yosemite Village for the location and price. The Awahnee was great to visit and eat a meal at, but too rich for my blood since we were only in our rooms to sleep.
Wait to do Tahoe for a winter week if you like to snow ski. They have about a dozen different ski resorts (including Squaw Valley and Heavenly), plus casinos, snowmobile riding, and the contrast between the snow capped mountains and the beautiful Lake Tahoe is a sight to behold. There are a lot of timeshares in the area, and you can easily spend a week here in the winter without becoming bored.