Universal is brilliant especially for teenagers. I disagree on the driving- you will be fine as you have 3 weeks. Flights here are not so cheap anymore and driving is easy-LA to Vegas in 5 hours. As long as you are happy to spend bits in the car it is much more economical to drive. San diego is perfectly pleasant and you will enjoy it I am sure- it is just a big leg on the end of the trip but if you get to see friends it will be worth it. There is an amazing hotel in Yosemite called The Awanhee- The Shining set designer used lots of its aspects to design his set...anyway it is pretty reasonable to eat lunch in there so that could be a nice treat-fabulous dining hall.....so you kind of access the hotel through dining there-maybe book ahead in the summer. Also get out early to get into the park each morning as the traffic really builds up. Let me know what Death Valley is like- yet to visit!! Have a great trip.