The St. Louis Zoo is the #1 free attraction in the United States. If the weather is nice, expect it to be packed and parking to be impossible to find. Recommend adding at least 45 mins - 1 hour for traffic expectations in and around Forest Park. Right now there is a lantern festival going on with Chinese lanterns installed around the Zoo.
The St. Louis Art Musuem has some wonderful pieces and is free to the public. The museum is very close to the Zoo and will also have parking and traffic. Please allow 45 mins to an hour for traffic.
The St. Louis City Musuem is one of the most unique locations in the country. It's not really a museum, but a 10 story jungle gym made of recycled materials. Please allow for plenty of time, dirty clothes, and don't injure yourself please!
Hey look, there's the Arch! For those of you who want to go see the famous St. Louis arch, we recommend actually going to the Old Courthouse in St. Louis. You'll get a better picture of the entire arch. The arch has a musuem at the base and you can go up in it, but please watch the time as it can be quite a line to get in.