There are many good summer day camps in St. Louis area. STLToday recently has a good piece on this topic.
Personally my old daughter has been to the JCC Day Camp, and Andrews Academy. We like both, with a bit preference over Andrews: for its after care, and overall quality. Note cost-wise, Andrews is a bit more expensive than the J (about $1,300 per month vs $220 a week for the J). We also thought about the Arts Camp at COCA too but the pickup and dropoff is a bit challenge (due to half day schedule). Note they do have 2 locations: the U city campus and the Whitefield school at Ladue/Mason. My daughter was very much interested in the arts, that’s why we signed up the art camp at the J when she was 5 (incoming kindergartener). This summer we are thinking again sign her up at Andrews (4 weeks Science camp and 4 weeks Outdoor camp). Both the J and Andrews do swimming once a day.
For the little ones, the summer camp at Hope Montessori (Mason road, Creve Coeur) is very good. We are thinking about signing up younger daughter this summer.
Last but not least, found a summer camp search website here.