One of this city's best-kept secrets is the Holcomb Observatory at Butler University. The observatory offers interesting and educational planetarium shows on Friday and Saturday evenings, and afterward you're allowed to climb the tower to gaze through the observatory's giant telescope (weather permitting).
Bonus: The planetarium shows cost only $3 for adults. That's a pretty cheap date.
At the moment, the planetarium show starts at 8:15 p.m. on Fridays and Saturdays; doors open at 8 p.m. The current topic is "Secrets of Saturn."
When we last visited the observatory, the topic was the Roman understanding of astronomy (tied into the art museum's Roman sculpture exhibit). It was a fun activity for us as a couple, but the families who were there also seemed to enjoy it.