Another place unique to St Louis and of very high iconic value . It's great to take pictures from the top. Another option: have your group stop by the arch on your way out of town for pictures outside the arch (this way you can visit something else for excursion). Check the weather first!
Science Center
You may find that this place is somewhat oriented toward younger people learning about science, but it's fun. It does have a really awesome Omnimax theater (half-dome screen that surrounds the audience--pretty cool) that has an amazing sound system. May want to get tickets ahead of time.
Creve Coeur Lake Park
This is not a main tourist attraction of St Louis, so it may be a more relaxing place to visit. No motorized craft (other than small electric motor) are allowed on the lake, so it is peaceful and quiet. Note: swimming prohibited.
Delmar Loop
Has many unique stores and restaurants.
IKEA
Shopping for furniture. Hmmm. Note: on Memorial Day weekend this place will be extremely crowded. You could spend a significant amount of your excursion time waiting in line (a.k.a a queue).