OLD SAN JUAN — “Take me to Fortaleza.” “No, take me to El Convento,” I tell the driver instead. With rum in the future, I figure it’s better to start higher up in the hills of Old San Juan. Rum is always better downhill. “Here’s OK,” I tell him, getting out in front of Ostra Cosa on the San Sebastián Strip, home to, most famously, El Patio de Sam, the onetime hangout of San Juan sojourners like Hunter S. Thompson. It’s known as a burger joint but it also serves some of the city’s best mofongo and some of its best chicharrones. Okay, so some of the best everything. But that’s tru...