Colombia is one of those places where you don't just see the sights, but also feel them, and while you might be inclined to spend your time exploring the culture and people on a bright sunny day, you'll be surprised at how equally charming the town is once the sun has set.
My wife and I went out to dinner one night, and to really appreciate the fact that we had an entire 4-days to ourselves without the kids, we toured the streets of Cartagena at night, avoiding any guilt we would have felt if the kids had been with us.