Anywhere where there are twinkly lights, a little water, and less people makes for a beautiful romantic stroll; I like walking all around Crescent Lake through Boardwalk and the Yacht and Beach Club Resorts. There are even some porch type swings out on the beach to sit a cuddle for a little while. Epcot is gorgeous at night, especially most of the WS pavilions. The beach at the Polynesian is perfect for a late night drink and a nice snuggle down by the water. Weird though this sounds, I find the Peter Pan ride, though short, to be romantic, travelling over London. If you hit the Gran Fiesta Tour boat ride in Mexico when it is not busy (which is rarely is), ask if you can have your own boat. The CM will often oblige and it is nice to be able to chat with your sweetie knowing no one is nearby to hear your sweet nothings! In addition, many of the resorts have little hidden gardens, walkways, and even carriage rides, all of which are very sweet and romantic. The further back you are in any park as late as possible, the less crowded and more couple friendly they are.
I hope those are some good ideas!