small but beautiful towns within distance of 150km from Warsaw.
Really, only two towns come close to meeting that description:
£owicz, which is about 70 KM west of Warsaw; and
Kazimierz Dolny, which is about 160 km southeast of Warsaw, which is quite scenic (and touristy in season for that very reason).
Toruń is about 260 km to the northwest of Warsaw. It's beautiful, but not that small.
Otherwise, there aren't a lot of places that I would characterize as "beautiful" and "small" in central Mazowia. Maybe Pułtusk, Płock, Piotrków Tybunalski or Ciechanów, or Żelazowa Wola with a side trip for a hike in the Kampinos Nationa Park.