I only know Columbia and St. Louis, but I agree that
--the STL zoo is awesome--if you go, the penguin house is the best part (imo, of course).
--The Arch is boring, and it's $10/person to get to the top.... save your money.
--The City Museum (in STL) is awesome and just a few blocks off 70--but I think it's in the $13/person range.
--15 minutes east of STL is Cahokia, which was the largest city north of Mexico back in its heyday... mounds, lot of (pre)history. Educational, great museum, great place to stretch legs.... They only ask for a donation, so whatever you want to give. Really, one of the most important archaeological sites in the United States, if you're into such things. A little bit off 70, but not too bad at all. My favorite place to see in the area, but I'm biased. :)
Columbia is, as someone said, a college town. If you just need to get out to stretch legs, there are a few spraygrounds in town (the closest one to 70 being at Stephens Lake park). Really, not much worth doing around here.
Between Columbia and St. Louis are a few parks here and there. The one that comes to mind is Graham Cave SP. You can't really go far into the cave now, though, but it has a lot of history, too. And places to picnic, get out some steam (walk/hike), etc.
Also, Six Flags west of St.L.... but never been to this one.
Missouri, apparently, recently won some award for having the best trails...
There is stuff to do, but google will probably help a lot... I've lived here for 3.5 years and still find myself wanting more... :)
Safe travels!