We took our first ever trip to DL last month, arrived evening of 6/9. We planned for 4 days to cover DL and CA. This allowed for riding all the major attractions multiple times. It also permitted us to plan our days exactly to our liking, experience many of the restarants, enjoy the resort pools, etc. We actually stayed 4 nights at Disneyland resort and 2 nights at GC. The 2 days we did not do Disney were spent at Knott's and 6 flags Magic Mountain. Our Disney days were a Thursday, Friday, Saturday, and Monday. Thursday and Friday were great from a crowd standpoint, Saturday was much more crowded, Monday was busy but not like Saturday.
Just like WDW, planning in advance made a huge difference. Remember the park is smaller so you can get from place to place faster. Using fast pass can make the day very efficient. And the two park's entrances are on a common plaza. If you stay at GC there is a resort only entrance for CA, directly across from the entrance to Grizzly River Run. We fast-passed the ride then went back to the hotel for an afternoon swim. While we were at the pool our fast pass time came up and we just walked into the park for the ride, already in our "wet" clothes. Just one example of how you can save time. You can also take the monorail from Tomorrowland to the middle of Downtown Disney, almost back to DLR.
You can do it in 3 days and see it all. We like to see and do everything more than once (we rode Matterhorn 8 times, CA Screamin 4 times, etc) so the 4th day really helped. As they say, DL has the same number of rides as MK, just in a smaller area.
Knott's is close by, 15 minute car ride at most. Knott's is a good park, it will take most of a day if you decide to go. Magic Mountain is in Valencia, about an hour up I-5. It is a thrill park pure and simple. You like coasters, you will love 6FMM. X may be the wildest thing I have ever been on, anywhere.
Hope this helps, your in for a great vacation.