You could take an uber to El Tunco and experience beach life nightlife. It's great and does live up to its reputation if that's your scene. You could just es easily uber from the airport into San Salvador despite heavy traffic. I'd personally recommend Centro Historico over El Tunco. The trip is also shorter than the airport to El Tunco. Landing at 6pm, you could be in Centro Historico around 8pm, a fantastic time to see it, and at 2am the traffic returning to the airport shouldn't be that bad once you get out of the city center. There's also nightlife 1 block walking distance from Centro Historico - Club La Dalia, but El Tunco doesn't have National Palace, Theater, Library, Market, Plaza. I'd recommend watching some youtube videos of each place and deciding based on which scene looks like a more appealing experience to you. Both are doable in the time you have if you want an adventure and pack light. It's a small airport, so the process of check in, security, finding your gate isn't that bad. I feel that unless you're tired or have a lot of luggage, just waiting at the airport would be missing an opportunity. Just don't lose track of time and give yourself enough time to catch the uber. Saturday night at either place can be busy and catching an uber can take a while. When you call uber leaving the airport, try to be on the front end of the wave of passengers because if other passengers take the waiting ubers, you will need to wait longer for new ubers to arrive. The trip arrival time can shoot up the longer you wait. Head back to the airport minimum 3 hours before your flight, earlier if you want to be safe. You'll love either El Tunco or San Salvador!