Kemper Museum of Contemporary Art lists free general admission and an evening opening on Thursdays. Free and late is a useful combination: you can go after work, look at a few things and leave without feeling you wasted a ticket. We have not seen the current exhibitions.
For October 5–11, the posted schedule is: closed Monday and Tuesday; Wednesday 10 a.m.–4 p.m.; Thursday 10 a.m.–9 p.m.; Friday through Sunday 10 a.m.–4 p.m. The Nelson-Atkins also lists Thursday hours until 9 this week, which makes Thursday the evening for both.
That Thursday, October 8, is also the date of a member preview. It has its own terms, and free general admission may not get you in.