Eight (8) of the world's largest items as certified by Guinness, they have the certificates to prove it! If you are in the area it is worth a side trip to go check out these amazingly sized items. In the summer on Saturdays between 10 AM and 2 PM they run a free trolley that will take you around to all the items. (Really worth the time to take the trolley as some of the items are not right in town or visible unless you know where they are.)
They need a new picture, the dragon is now wearing a top hat! This is a cute, quick little side trip and worth the time.
The Missouri Botanical Gardens (also sometimes referred to as Shaw's Garden) is a fascinating and worthwhile visit when in St. Louis. The plants are marvelous and wandering the grounds reading the names of every flower and tree is enlightening. The desert house and the climatron are wonderful ecological microcosms. The highlight of any trip though, in my opinion, is feeding the hundreds of koi in the lake in the Japanese garden section
There are major renovations going on in and around the arch right now (July 2016). While you can still go up in the Arch the museum is closed. The grounds around the arch are torn up and not accessible. You buy tickets at the old courthouse, about a block from the actual arch. All that being said, it is still worth the time and effort to go up in this engineering marvel that is over 50 years old now. The view over the city and the river front is wonderful (and even more impressive if you happen to be up there when a storm comes through like we were).