I love Old San Juan!
I've found great walking tours in books from Frommers and DK. What I like about them is that they are not too bulky (Frommers newest editions are much thinner) while still including a lot of description and history. That said, I've yet to use either in Old San Juan, but they have not let me down in other cities in North America and Europe.
Another essential: comfortable walking shoes. Break them in for a few weeks before you go.