Plaza Las Américas is the largest shopping mall in the Caribbean and the busiest in Puerto Rico — over 300 stores spread across a complex in the Hato Rey financial district. The anchor tenants are the standard US department stores (Macy's, JCPenney, Sears for many years) plus a movie theatre, a food court, and most of the international clothing chains.
For travellers it is less interesting than the small shops in Old San Juan, but if you need a US-style consumer-goods stop, this is the place. Tax is low (Puerto Rico has lower sales tax than most US states) and many travellers stock up on items they can carry home duty-free. About fifteen minutes by taxi from the historic core.