Our first attraction to visit in Poland was a place said to be the "Longest military defensive system in the world" and "the largest military basements in Europe": Międzyrzecz Fortification Region. It was well worth the visit: it's an extensive network of underground tunnels, built 1934-36, which housed railway systems, war equipment, and even factory facilities. For non Polish speakers there is an audioguide, although you are required to follow the Polish tour (which is difficult because the tour guides commentary and the audio guide are at different paces).
On our first day in Poland we didn't actually make it to the underground attraction before closing. But nearby, where the road ends on a lake, there's a pleasant campsite with lake swimming and kayaks to use. It was a heatwave across Europe and we'd driven a long way that day without air-con, so we very much appreciated an evening swim before heading underground the next morning.
Here's a picture, we recommend visiting the campsite and the Międzyrzecz Fortification Region.