Hi there!! I believe there is a company called "Old Town Trolley Tours" and they have a day tour that takes you to all of the cool places to see in San Diego. I'm pretty sure you can catch a trolley at the start of the tour and get off at various spots...i.e., The San Diego Zoo...and then catch the next trolley that comes along to continue the tour. I also believe it takes you over the Coronado Bay Bridge and to the Hotel Del, Old Town, etc...It might be something to look into. Enjoy!!
Brandywine