Free days at the St. Louis Arch. Entrance fees are waved Apr 20-26. Nice timing!
From that website:
The fun will begin with a series of Fee Free Days from April 22 through April 26. On those days, the National Park Service will waive its park entrance fee.
At Jefferson National Expansion Memorial, a $3 park entrance fee is included in each adult “Journey to the Top” of the Arch ticket and/or movie ticket. During the Fee Free Days, all “Journey to the Top” of the Arch tram passes and/or movie tickets will be discounted by $3.
The tickets to the top will be $7 per adult and $2 for kids 15 & under per their website, the free admission is just for the museums at the bottom.
Having been several times I reccomend going to the top of the arch, it’s really fun, unless you’re claustrophobic… Then it’s not a really good idea.
Speaking as someone that does not enjoy tight spaces or heights, I am glad my team encouraged me to go up last year, glad I went up, but now that I have done it, I will not do it again! 
I would definitely recommend it though, to everyone!
Last year the trip up the Arch was discounted for every robotics team.