Other than renting a dinghy, they usually require some type of certification, i.e. US Sail, or ASA. Harbor Sailboats has basic keelboat classes every weekend, and their boats are in nice condition. They have a sale going on now that you could check into. Great classes with plenty of on the water time while on beautiful San Diego Bay. Their basic keelboat course uses Capri-22's
http://harborsailboats.com/
Seaforth boats are rode hard, and put away wet. We went to charter a C-250 from them and it was filthy, and you couldn't go below the head smell was so bad. I couldn't believe the condition of their Capri-22's and Capri-18's either sitting at the dock.
Too bad you'll be here in May. May and June historically are the worst months we have along the coast with fog. They don't call it the "May Gray" and "June Gloom" for nothing. So bring a sweatshirt!
Check out Harbor Sailboats, and enjoy San Diego.
Don