A nice dinner (someplace you don't see kids) and a movie sounds nice. Or my husband and I love to go to the symphony or jazz concert. If you want to do something, try dinner and putt putt golf or even bowling. Enjoy your time before kids!!!
Whatever you decide, take your time when you are out. Once the kids come home, I find that when we go out we are usually checking the clock a lot because we want to be home around the time we gave our babysitter and to avoid a huge babysitting bill.