I think the most appealing aspect of Old San Juan is the architecture and design of the city. Residential apartment buildings have been converted to designer boutiques or little B&Bs, and interior courtyards invite hungry guests to relax and unwind in the shade....
I've been to this area multiple times and still I manage to find a street I've never wandered down. Some are more popular than others, in particular Calle San Sebastian, which hosts eateries, bars, and shops, and Calle San Francisco. At the corner of this street is my particular favorite, Franky's antiques. It's in a four-story building, loaded with vintage goodies. There's so much to see, one trip is not enough.
If you want to shop, consider wandering all morning and into the early afternoon. You'll be delighted by all the unique shops. Then, take a break at my favorite restaurant, Triana, for Spanish-style tapas and their famous sangria. It's a must-stop place any time I'm in the old city.
You don't need a car to get around. Just park at one of the garages and explore by foot. There's lots to see and do in Old San Juan.