Many many times over the years we have taken our children, nieces, nephews, grandchildren, and adults to the St. Louis Gateway Arch. Everyone enjoys it.
You can see the Arch from miles away. It sits near the river and is beautiful day or night. When you go in to the Arch, you must go underground. There is a movie room that you can watch a film of the making of the Arch that is fascinating from a architectural view.
There is also a museum to go through. For souveniers there is a nice gift shop.
If you are not afraid of heights and are not claustrophobic, then you must go to the top of the Arch. The view from the top of St. Louis is awesome.
St. Louis Gateway Arch - St. Louis, MO is recommended by Barbee