Plenty to do, the arch is cool (although maybe not if you have claustrophobia). The bar area people are mentioning is Laclede’s landing.
There’s also the Science centre (free), the Zoo (free), the Art museum all in the Forest Park area, Anheuser Busch brewery tour (free), City museum (not free, but they’ve got slides made of rollers that go down 3 floors).
Ted Drewes on Route 66 does great frozen custard.
Go buy cheap outdoor stuff at REI or Galyans (although think this is Big Dick’s sporting goods now), although dollar rate not too favourable at the moment.
Avoid East St Louis (other side of the river in Illinois)