I’ve never found the need to rent a car when I am in San Diego because the public transportation system has always been very useful.
I have used the MTS trolley and have nothing but great things to say about it. It’s easy to navigate and the self-service ticket kiosk makes buying tickets very simple. At one station, I even encountered transit patrolmen who were willing to assist with ticket buying.
The trolley lines are color coded for easy reference.
The Orange Line is the one that stops at the Gaslamp and the Convention Center. It loops downtown and then goes northeast to Gillespie Field, along with the Green Line.
The Blue line service points north and south. If you are coming from the North on the Blue, you can transfer at the America Plaza to the Orange line. From the South you can exit the Blue line at the Transit Center and walk or transfer to the Orange line.
If you are nearest the Green line you’ll have to take that South and then transfer to the blue line and then the orange.
The Red line runs during special events only, including SDCC. It runs from Qualcomm Parking to the Convention Center.
One way fares are $2.50. Day passes are $5.00, a great deal if you plan to do a bunch of running around. The day passes also include use of the extensive bus system (with exception of their Premium Express and a few other routes.)
Normally the trolley runs from about 4:45 am to after 1:00 am. They will have special hours during SDCC; a link will be on the official SDCC site, and the MTA site. The site also has great maps and information about bus service and other regional track transit services. You can view all options for trolley and bus service to and from most major attractions.
Paying for fares is quite simple. Ticket kiosks will take either $1 and $5 bills, credit/debit cards, or a combination of both.
The San Diego Metro also has a mobile version of their site which can help you plan trips to and from the convention center and the surrounding area.
In 2012 the SDMTS updated their cargo policy, restricting the size and number of large cargo items each person can carry. These rules do not apply to any airport routes but if you plan on traveling with a large amount of baggage on other routes, please check their web site for detailed rules.
Trolley service during the con will most likely be crowded, so be ready to stand!