Good for you!
My husband and I just returned from a Labor Day Weekend birthday trip together, sans children. We have been married for 14 years and our oldest is 10, and we had only spent one night prior to this away from our children. It was HEAVEN! We had so much fun. And although I missed the children and thought about them alot, I relished in the time my husband and I were able to splurge on ourselves.
So what was the most romantic thing we did together? I'd have to say that it was when we watched the Kiss Goodnight at the Magic Kingdom after FITS. We sat in one of the benches right in front of the castle and watched it turn beautiful colors. Then we slowly walked down Main Street, soaking in the last few moments.
Another romantic moment happened right before Illuminations, the night we arrived. I'd grabbed my Happy Birthday button at Guest Relations and was wearing it while we waited to get onto El Rio del Tiempo. The CM loading the attraction noticed it and gave us a boat all to ourselves! That was very romantic. I'll bet if you also went right before Illuminations (when it is very slow), you could ask for a boat to yourselves, explaining your wonderful situation. It couldn't hurt to ask!
Have a wonderful vacation together.