This sounds like a fabulous trip. With 21 days, I would break the trip roughly into 3rds - a week in northern CA, a week in central CA, and a week in southern CA - adjust slightly to add more time to one or the other based on your interests, but that is roughly how I would divide the trip. If you want to do a purely coastal trip, that would be wonderful - see the entire CA coast - and you have time to really do that well. Or you could break things up a bit and head inland to see Yosemite as well and/or the wine country north of SF. There are other wine regions on or near the coastal route too. The possibilities are almost endless :)
I agree about reposting in the California forum though rather than Sacramento - you will get more input that way. Also, give us a little more info about your interests so people can give you better advice - no use rattling on about wineries if you don't care for wine, for instance :) Also your travel style - budget, luxury, etc. and how active and on-the-go you like to be.