Hi RoadTrip-Pros!

first: sorry for my english... its not perfect..

i read some Post but could not exaclty find what i was looking for.

me and my colleague are flying to Seattle on 23. August. we Pick up a Rental-Car in Seattle and return it in San Diego. We are flying back on 14. September from San Diego.
Therefore we have 21 Days for our Roadtrip!

we are both 22years old, from Switzerland and we are planning to see as much as we can in this three weeks! ;) i hope it is enough time for this trip!!???

As we heard alot great things about Vancouver we plan to go there and then all the way down to San Diego..
we want to stay 2-3 Days in San Francisco, Vancouver and LA.. or any other suggestions for interesting / nice citys on this route? we want a good mix of everything :) Fun (nice places to go out in the evenings... party.. not every evening of course..), things like National-Parks (perhaps Redwoods?) and Hollywood / Universal Studios..
what would you suggest? Any "must-see" places? Tips?

if we have enough time we may want to visit tijuana at the end of our trip.. good idea?? or too dangerous? how far is it from san diego?

Thank you all!

Michael