14 places to visit from north to south
This is a fantastic trip you have planned. I think you can make it in 2 weeks, provided you very plan well. 3 weeks would be more comfortable.
There are tons of campgrounds along the way. You may want to consider booking ahead as that route is popular (summer and all) and some campgrounds can get full. A place I find useful to do my research is http://www.CampingRoadTrip.com I like it because CampingRoadTrip.com has an easy way to select a city, then a radius function, so you can see campgrounds within 25 miles of what ever city you plan to be in that night.
Cities to stop in, well, hard one, but...from north to south along your route, my picks would be:
1. Seattle (WA) - do the underground tours of the city which is super interesting; catch a live band somewhere; fish market is also facinating
2. Cannon Beach (OR) - grab a pizza (great pizza there for some reason!) and sit on the beach and eat it, watching the sun set
3. Oregon Sand Dunes (OR) between Winchester City and Florence - worth doing a sand buggy ride over th dunes, and also looking at the lighthouses in the area
4. Cresent City (CA) - Redwood National Park is beautiful, and some hiking for sure
5. Pt Reynes National Seashore (CA) - has great hikes and jagged coastline, bring camera and water
6. San Francisco (CA) - DON'T MISS ALCATRAZ, there is also Haight Street for some edge / counter culture, the usual Golden Gate Bridge, Chinatown for dim sum, that crooked street etc. A friend of mine hired a mini car thing which she swore was the most fun she's had on vacation!
7. Carmel by the Sea (CA) - quaint town, good to stop for lunch
8. Pebble Beach (CA) - must if you are a golfer, also there is a wildlife sanctury near there (can't remember the name) but you can see seals and all types of creatures basking on the rocks in the ocean
9. Julia Pfeiffer Burns State Park (CA) - one of my favorites for hiking and camping in that area is great
10. Hearst Castle (CA) - worth paying the money and seeing this thing! You'll be impressed by the stories alone. There is a campground around there at Moro Beach or San Simeon (if I remember corrrectly) that is great
11. Santa Barbara (CA) - nice town, worth a stop and stroll
12. Santa Monica (CA) - gorgeous and interesting beach, you'll waste an entire day here and be perfectly happy
13. Legoland California (CA) - fun for big kids!
14. San Diego (CA) - fun city to hang around, but go to Coronado Island, sit by the sea and have a beer.
Have fun on your trip!