2 days, 3 nights in Houston

My family and I (3 adults, 3 children aged 9, 7 and 5) are going to be in Houston for 2 days and 3 nights the last week of the year. We plan on going to the NASA Space Center one of those days, but we are not sure if what to do on the other day. The two leading candidates are the Zoo or the Aquarium.

I was curious to hear peoples input on those two choices, or other suggestions for things to do for a day as well. We live in North West Arkansas so we are not used to cities the size of Houston, so we like to take advantage of that.

Edit: we are staying just south of Downtown, about 20 minute drive from the NASA center.