Forest Park is a real gem. It's as nice an urban park as I've seen in the US, and rivals some of the really cool parks one can find in Paris, like Buttes Chaumont.
as a kid not from St. Louis, I remembered doing the Zoo Cardinal game weekends and having a picnic at the park. The size of the place was overwhelming for a small town kid.
I'm a big fan of the trail system -biking running walking segways.
The park has it all- the institutions ZOO, Muny, Art Museum, History Museum
classic attractions like the golf courses, the Boathouse, Steinberg Skating Rink, the Jewel Box, Davis Tennis Courts
history -1904 Worlds Fair, Olympics, Lindberghs welcome home, etc etc.
Numerous scenic spots - Art Hill, Grand Basin, the Post-Dispatch Lake, etc.
I normally hesitate to Rave on good places (especially if they are free), on account that their popularity dooms them. But Forest Park has proven it can handle large crowds enjoying all its amenities.