Try and get a night-time flight
KT,
If possible, try and get a night arrival flight, avoid sitting over the wings and get a window seat (ideally on the left side of the aircraft -- depending upon the approach pattern in use when you land...) When you fly into Las Vegas at night, it looks like you are flying into Oz. The green of the MGM, the blue/red of the Polo Towers and the rest of the LV strip display is really pretty cool!
Depending upon the weather -- that stretch of air space is also a bit bumpy as you will pass over five significant mountain ranges.
Personally, I don't think it is reasonable to try and hit Yosemite or any of the parks inland from the coast in your time frame. If it were me, I would plan to spend the entire time exploring the coast as you go south. The coast highways are slower moving than the Interstate highways and will easily take five days to drive from Seattle to San Diego. If you pause for a day or two as you go at some of the more memorable places and towns, it will certainly take all of the two weeks to make the drive. This should not be considered a "speed run". For a sample of places to stop and explore do a keyword search (select all topics -- gray button top of this page) using coast road trip. You will find a legthly topic thread about such trips taken last year.
Mark
Mt. St. Helens is an iffy one
Even though your trip is not until the Spring, Mt. St. Helens is awake and active and may be then too. While it will make your trip more memorable -- you will need to be somewhat flexible since no tour operator can predict if the roads and visitor centers will be open.
I am sure that you will hear from poster Judy about tours in this area one of these days.
Mark