How to maximize 18 hour layover in Barcelona, Spain??

I would find an airport hotel so morning is easy. Spend the evening in central Barcelona. I think there is a bus that goes from the airport to a stop near the Ramblas. Consult a book on Spain for travelers. Rick Steves has such a book.

I would recommend Hotel Jazz which is a nice 3-star hotel,with rooftop bR and pool, lose to Plaça Catalunya. Take the Aerobus; you can get off one stop before Plaça Catalunya at Universität which is closer by one block. Check in and head up to the roof for a skyline view and maybe a beverage. Then walk to Plaça Catalunya and down Las Ramblas a short way to check our the scene. Watch out for vendors and pickpockets, and do not try to eat along here.

We liked several tapas places in Eixemple which is the other direction from Las Ramblas, but close. I won't suggest those, as they did not take reservations, and you could spend a lot of your precious time waiting. So try to find a recco for a nice place close to Plaça Catalunya, say within 5-6 blocks, and walk there. Barcelona is very walkable, and for places in the Eixemple you may walk past Casa Mila and other Gaudi sights on the way.

Another vote for the fabulous Hotel Jazz! Ditto on the walking tour of the area. In addition, for a gorgeous city view go to the top of El Corte Ingles Department Store. This is on your way to Las Ramblas.

If you really do mean 'maximize' then don't spend your time or money on a hotel at all.

Go directly from the airport to the evening show at the Palau de la Música Catalana
http://www.palaumusica.cat/ca
followed by dinner at someplace like Merce Vins in the Gotic
and then go to a bar/club/party near the University like La Catedra (open until 3am)
or the People Lounge
and then take a stroll in the dark around the gardens and fountains of Montjuic
and then get coffee/breakfast around Plaza Reial
and then get back to the airport...

Avirosemail certainly gave you a maximum 18 hours. If you’re looking for something a bit more conservation but that includes much of the “night sights” and even a little sleep (in a bed), I offer the following

• When you arrive at the Barcelona airport, store your luggage, taking with you only a change of underwear, a shirt and toiletries (perhaps in a small backpack), then
• Take the Metro L9/L1 to the city
• Pre book a hotel near a Station on the L1 (for a quick trip back to the airport the next morning).
• Drop off the backpack then
• By all means attend a performance at the Palau de la Musica Catalana (unless it’s dark, if that’s the case then)
• Pre-book a Jazz performance at Casa Mila “La Pedera”
• If the performances at the Palau or Casa Mila end in time go to the Font Magica for the "light show" then
• Dinner at one of the roof-top places at the Las Arenas Mall (perhaps not the crème de la crème of Spanish cuisine, but pretty good and a great view), then
• Clubbing till sometime (?) the next morning, or, more conservatively
• Take the Metro L1/L2 to Sagrada Familia Station
• Walk around the fabulous edifice then
• Take the Metro L2/L1 to Glories Station
• Walk around the Torre Agbar, then walk to/through 22@ (Poblenou), then
• Take the Metro L4 to Barceloneta
• Walk around the area, perhaps a night-cap then
• To your hotel for a few hours sleep, then
• Breakfast then
• Take the Metro L1/L9 to the airport.
  

Rooftop dinner does sound like an even better idea, weather permitting!

I took it as understood that you leave your stuff at the airport, and that wandering in the dark on Montjuic means you get to see the sunrise. Maybe some early joggers in yoga pants, too.