After decades of neglect, Casco Viejo is finally being revitalized. New hotels and restaurants, some quite elegant, are occupying the centuries-old buildings. Iconic landmarks like El Arco Chato (Flat Arch), which may date to neighborhood's founding in 1671; the 1798 Metropolitan Cathedral; and many other pretty plazas, palaces, markets, and gold-gilt churches have been refurbished, as have the narrow streets, draped in flowery French balconies, which connect them. Well worth a wander.
Take a taxi, even during the day, unless you're quite comfortable with impoverished areas such as those that surround the old town.