The distance from San Francisco to San Diego is:
502 miles and if you drove without stopping would take about 8 hours (perhaps longer or a bit shorter based upon traffic (difficult from Los Angeles to San Diego especially at times) and how fast you drive.
The drive from San Francisco to Monterey is coastal with pretty ocean scenery along Highway 1 (PCH). Its usually pretty easy drive unless accidents or major traffic.
From Monterey to Cambria, the views are gorgeous....if not too foggy out (if it is foggy skip this part of PCH), but the driving is difficult....lots of curves, turns...and difficult going. Again, its beautiful but difficult driving and can easily tack on 3-4 hours more to the drive if traffic, stopping to see things, etc. are involved.
There is a way to get around it....and that is by taking the Monterey cutoff to Highway 101 and then driving Highway 101 down to Pismo Beach where you catch up at Highway 1 again.
Note: You can also cut over at Cambria turnoff if you want to see Hearst Castle on the way down.
Highway 101 (PCH) at that point is rolling hills, or beautiful ocean between Pismo and Santa Barbara.
From Santa Barbara out there is some ocean, followed by hills to climb on your way to Los Angeles. In LA its typical city freeway frenzy (someone from Boston probably knows how that can feel). Once you connect however with PCH (Long Beach on its gorgeous ocean scenery again. Beautiful driving for most of the way to San Diego....Huntington Beach, Laguna Beach, etc.
For a part before you get to San Diego it can be not so scenic for about 45 minutes on your way down to Del Mar/La Jolla which again it is scenic ocean.
The best way to drive PCH would be one overnight (or more for a more leisurely drive) from San Francisco to San Diego so you can see some of the sights.
Yes - you can drive the whole thing in a day....but to really enjoy it takes at least 2 or up to a week.
Is your flight back to Boston out of SFO? If that is the case... you will spend a good portion of your free time driving. But if the drive itself is the goal... then it is possible.