If the forecast is looking bleak, why not spend some time at Casa del Rey Moro African Museum? You'll be surprised just how much you can learn about the past here.
You'll have no end of sightseeing choices when in Old Town San Diego. Casa del Rey Moro African Museum is a good place to begin here, but there are plenty of other interesting spots to consider. San Diego Zoo, Balboa Park, and University of San Diego are near the collection of historical artifacts and will round out your day nicely.