You don't specify your level of play, but there's decent club level tennis (say, 3.0-4.5) to be played with the san diego tennis exchange (http://sandiegotennisexchange.com/
) and the north county tennis network (http://www.tennis-sandiego.com/)
. Both are drop-in doubles.
Barring that, there's morley field in balboa park. The level of play there is generally pretty good (4.0+), but there's not really a drop-in scene - you'll need to hang around and invite yourself onto a court there with someone looking to hit.
Good luck. SD is a great place to play.