I like San Angel Inn for dinner too. It's a very pretty place. Another thing you can do is to take a fireworks or sunset cruise (I think prices start at $120 for the cruises). You can even charter a yacht for a dinner out on Bay Lake or the Lagoon.
Cirque du Soleil is an amazing show, and much more adult-oriented.
Dancing at Pleasure Island is also romantic and fun.
Strolling through the World Showcase after dusk, when all the lights come on, is also romantic. It's basically empty at that time, and you can stop here or there to eat appetizers. From some of the restaurants you can watch the Illuminations fireworks after dinner.
Visit the Animal Kingdom Lodge. Have a drink in front of the fire at the Wilderness Lodge. You can also go horseback riding at Fort Wilderness, though I think this is only in a group environment.
Have breakfast on your balcony, if your hotel offers room service. I also read somewhere that you can have a dinner set up for you on the beach (you might need to stay at the Polynesian, GF, or Contemporary for this one).