The Maxo Vanka Murals are a hidden gem, permanently on exhibit in this little church, close to the river in Millvale.
Maksimilijan Vanka was a celebrated Croatian painter who emigrated to Pennsylvania during the turmoil of the 1930s. He was invited to paint the walls of the St. Nicholas Croatian church and given artistic license to not be shy about his politics. So instead of the typical Catholic iconography, you see very progressive images of the Croatian experience, complete with coal mining and depictions of World War I. These murals are flat out awesome.
You certainly could go to the church for regular mass, but you'll get a better look at the murals if the docent can point spotlights into the corners of the church. It's my understanding that Serbs are welcome too, but fascists should stay at home.
Docent-led tours on Saturdays: 11:00am, 12:00noon and 1:00pm (as of this writing, but check the website). Free admission (but bring a donation, for cryin out loud). If you time things right, you can get some baked goodies from a little blue-haired Croatian woman in the church basement.