The Slovak National Gallery was also open late- it was free that day too because they were changing the exhibitions and only two rooms were open. From the website it seems like there’s a lot of interesting stuff in the museum, and it’s a pity I didn’t get to see it, but I enjoyed the small section I did get to see.
This room was turned into a library using the personal collection of Slovak writer and translator Kornel Földvári (Slovak translator of Stefan Zweig amongst others). 
This room has the archive of Slovak artist Julius Koller
Upstairs was a photography exhibition by Anna Daučíková. I feel like I missed a lot of the exhibition because the text panels were in Slovak, but that’s really my problem for not knowing enough of the language.
The atrium is open from the ground up to the third floor. I got some serious vertigo taking this photo from a balcony on the top floor.