If you haven't been to Pleasure Island....you have to go! There is a club there for every taste...even if you don't drink...it's fun to visit the clubs and watch the various shows....both planned and unplanned (read: people watch). Can you say JELLO SHOTS!!
If you want to stick to the Downtown Disney area you could go over and shop (if you feel like it) and go to Fultons, Bongos, or House of Blues for dinner....depending on your taste. We like these three but there are even more choices.
If you want to go to a special place for dinner, try the California Grille on top of the Contemporary. Besides being just about the best restaurant on property...the view is worth the trip. Time it right and you can see the fireworks over Cinderella Castle. Even if you have a drink and appetizer and move on....this is fantastic. Watch the Electric Water Pagaent, take the monorail (ask to ride in the front car), and go to the various MK resorts to check them out. If you do this, make sure you take one of the boats across Bay Lake....people who don't stay at these resorts don't realize they are there and its fun and unique to do.
Or how about eating and drinking your way around the World Showcase in Epcot. Start in England and try an appetizer (and matching libation) and just keep on walking. Without children you can stop to enjoy the performers, parades, movies, food, drinks, shopping, etc. and then find a seat for Illuminations. In the past, before we hit our forties+, we would then take a cab from the Boardwalk to Pleasure Island to end the evening. And hey, if you have to go to the Boardwalk anyway....stop in Jellyrolls for some fun sing along and comedy!!!
Hard to decide what I would do if I only had the one night...but these are some things to think about. While I don't espouse commando disney....if you only have one night...try to fit in as much as you can!!!!