The arch is closed until "late March" - hope it's open for CMPS because it is fun to visit. They sell out every day when we are there so purchase tickets in advance. The history museum located within the base is fantastic (and free) if you get the chance to go through it. This is a national park so security measures are in place and screening lines can be long. Plan for it so you don't miss your reservation time to go up. The "pods" that you ride in for a short time are small - some people may feel claustrophobic. The park that surrounds the arch is a nice place to get away and relax.
http://www.gatewayarch.com/
If you have time, visit the Union Station. It's now a hotel but also the home to FLLJr and FTC Championships. During the evening the lobby has a wonderful projection show on the ceiling. There is a Hard Rock Cafe located adjacent to the hotel, near the metro stop. There are also shuttle buses running between venues daily.
If your mentors have time, visit the NEMO (Non-Engineering Mentor Organization) meeting on Friday between 10am and noon in America's Center, room 255. Great opportunity to network with other mentors and relax for a short time in a quiet space!
And if you are flying in, note that the airport is located some distance from downtown. You may need to arrange transportation or take the Metro to your hotel.
Have your team members track their walking mileage - you will be amused and amazed at how many miles you will put on during the event!