Ticket sales are nigh for Nerd HQ's Conversations for a Cause. First-timers, take a look at their site and some conversations from previous years and you will see why these are so popular. If you just want to focus on SDCC this year, that's understandable; however Nerd HQ offers some fascinating content and it's within walking distance. If you have a gap day, this should be high on your list.
Watch closely for announcements on ticket sales because that should be happening very soon. Also, consider their app.