There's an almost insane amount of merchandise that you can buy at the festival, but there's no end of free swag on offer from the stalls in Hall H. Warner Bros. has even sponsored swag bags for you to fill with take-aways from the festival. Snaffle yourself free stickers, magnets and pins, or win contests for collectors toys and books, limited edition posters, and wearable merch. Bring a phone charger and extra battery pack, and don't forget that many vendors do not accept card, so bring cash.
It's a good idea to eat breakfast at Comic-Con San Diego hotels before arriving at the Convention Center, as queues in the mornings are long. You can bring food with you, which might be handy as there aren't as many vendors as you might think. You can also leave the convention for lunch. Bringing water with you is a good idea.
If you decide to dress up as your favorite fictional character, expect to be asked for photos. This is especially true if you have put a lot of effort into your costume, which is recommended, since standards for this grown-up dress up are world-class, and the outfits make the event.