Well my hubby and I have no kids.. but sometimes we find the rare, whole day, that we both have off with no plans.. And we are at a loss too! It is so rare for us to have a nice long day together with no obligations, guests, or plans..
We take rides. We go drive along on some backroads, seeing the sites, trying to get minorly lost.. it is fun If you're in the city, go visit the country. If you live in the country, hit the big city. Or maybe re-visit a spcial spot that you two frequented when you were just getting together?
Jo's ideas were great as well