For me, it would depend on where I was staying. If you're on the north side of town, I'd probably skip the Butterfly House since it's on the far south side in Sertoma Park. It's a so-so attraction. The butterflies are pretty and the room they're in is like a little tropical oasis. On the downside, it's small and overpriced. Also be warned it's hot inside for the butterflies, so don't go in there wearing a sweater.
If you do decide to do the Butterfly House, the Outdoor Campus is right across the parking lot. It's more for kids, but it's free and has trails for walking and jogging if you feel the need to do this after sitting in the car.
Downtown is Falls Park and, if you only have time for one attraction, this is the one I'd pick. The falls are really roaring right now after all the recent rain. There's a free observation tower with your choice of steps or elevator. There's also a little cafe in a old building overlooking the falls.
Other attractions which I have not visited are the St. Joseph Cathedral (recently renovated and also downtown), Old Courthouse Museum and the Pettigrew House. I don't feel comfortable giving an opinion on those, so you may want to check out the TA reviews and see what people are saying.
I would skip the Washington Pavilion if you don't have kids with you. It's basically a hands-on museum for kids, small art gallery and Imax theater.
Basically, in my humble opinion, Falls Park is the only truly "must see" attraction in town. I hope I don't offend anyone with that statement.