The borough farthest to east in San Juan, Rio Piedras reminds me a lot of my beloved Brooklyn in New York.  Like Brooklyn, Rio Piedras was a fully functioning "sister city" (founded in 1714) until it was annexed by the capital in 1951.  As such, Rio Piedras still has its former downtown or "Centro" intact, which is the place to be among the young on Thursday nights.  Home to the University of Puerto Rico since, 1903 its distinctive clock tower is a landmark of both the neighborhood and the campus.  It is also home to the city's botanical gardens, a renowned masterpiece of tropical plant conservation.  But perhaps my favorite part of Rio Piedras is Paseo de Diego, a rather bedraggled outdoor mall that has seen better days but still offers everything from produce, to jewelry to auto parts along its sidewalks.  Not to mention incredible street food made from braziers and gas grills along the way.  But if street fare is not your thing, go into the market where the food court offers meals so plentiful you'll be eating them for three days.  Ah-- Rio Piedras!