When I went a couple of years ago one day was a Slavic Cultures Festival, too, and there were Croat, Czech and Ukrainian booths in addition to Polish. I had some good home-made Ukrainian food and pastries but the website only mentions Polish food. I hope there's something other than Polonia to sample.
Modest entrance fee, free parking, not much of a traffic hassle; the only problem will be if it rains and the vacant lot where you park is muddy, I guess. It's right down the street from Polonia.
I think I'm going to try to go on Friday.