Well, depending on how long it takes you for dinner, you may be able to see Illuminations, which usually shows at 9.
Afterwards, you have a few choices of where to go and what you want to do. As it was said before, you could go clubbing at PI, if that is more your scene. There is also a movie theater at DTD if you wanted to catch a late show together. You could stay in the Epcot area and go to the Boardwalk to walk around or there are a few bars here as well. Not sure where you are staying, but some resorts have bars or nice areas to take a romantic walk around outside.
Also, check the times of the fireworks at Magic Kingdom. You could go to the Poly and watch the fireworks from the beach there. Even though there are quite a few people around, it feels secluded and romantic.