Hillade, there is lot's to do in and around San Diego and San Diego County. You could also fit a visit to the Grand Canyon (~500 miles), one-day drive out, one-day drive back and 2 days camping. Grand Canyon plus flying to/from San Diego would account for 6 of the 10 days. It would be hard not to fill in four days in and around San Diego.

If it isn't too hot, or if you are the hardy sort, you could camp one night on the drive to or from GCNP, at Joshua Tree National Park. Doing that would make it a very manageable 5-day trip to the GCNP, plus your two travel days by air.