The City Museum isn't "cheap", but it can be an all-day thing, and is an AMAZING place for anyone from 1 to 100. My parents, who are in their 60s, went there w/my husband and I when we lived in StL, and I've never seen them let loose like kids before! It's got tons and tons of great things to see, and you can climb thru the walls and cielings. It's set up like a great big jungle gym playground both inside and out. The guy who put it together is a genius. I would say, that's a must-see, but save it till you have a whole afternoon or more. The kids aren't going to want to leave!