If you want to be a bit more adventurous you could grab a cab and head straight to Old San Juan. If you get there early enough in the afternoon you may have time to tour a fort and check out some of the great architecture in the daylight. Then, being a Saturday night there would be plenty of options for bars and restaurants.
Nothing wrong with sticking around the airport though. Metropol is a great restaurant and the El San Juan should be hopping that night. Make sure to have a decent outfit handy if you want to hang out at the El as they enforce an evening dress code.
Matt
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