Our Lady of Grace Grotto
In case you don't know what a grotto is, it's a bunch of little bitty gems/stones/rocks all kind of glued together to make a huge building/altar/whatever. There seem to be a lot of them in Iowa. Our Lady of Grace Grotto is beside St Mary's Catholic Church, and its a lovely little rest stop before you leave the city. It has a little stream and bridge and some really pretty sculptures, plus a religious shrine type thing (hope I'm not using that term incorrectly here) that tells a story when you press a button. It's quite a nice side trip, I highly recommend it!