£owicz springs to my mind...I think especially they have these beautiful processions, and there is also the skansen there,
One of the few towns in the area that you can spend a whole day in (along with Kazimierz Dolny). Most towns in central Mazowia may have one or two things worth seeing, but very little else.
I used to live in £owicz, and the Corpus Christi procession is worth seeing because of the folk costumes, as well as the museum and the skansen. The main street, Zduńska, has a very pleasant atmosphere. Not to be missed are the park in Arkadia, which is absolutely gorgeous, and the palace in Nieborów, both right outside of £owicz. You could actually do two days in £owicz.
Also, on ul. Mostowa, right off the square behind the museum, is Podsędek meat store, which has the best sausage in Poland, and next door is a shop that has the best bread in the world (no exaggeration). Buy some myśliwska (sausage) and get a loaf of this bread and you will be in heaven. It's worth the trip to £owicz just for that.