Go play on an architectural playground for all ages! It is both indoors and outdoors.
Go hike the trail where Jess and Charlie had their first date!
Go up in the arch! Or just visit the arch grounds and enjoy the museum at the bottom of the arch. Recommend making reservations in advance if you wish to go up in the arch because they tend to sell out on the weekends.
Go Learn Stuff.
Go get some frozen custard!