The castle is only open to visitors on Sundays; a permanent exhibition on its history and a changing art exhibition are open daily. Both are small but quite nice. The Schinkel Church has varying opening hours, as it is maintained by volunteers. The park is always accessible and a lovely place to rest. There's also a restaurant in the distillery with very good, upscale cuisine, although it's a bit more expensive.
Castle tours The ground floor is open Sundays from 1 pm to 6 pm during the calendar summer time. Half-hour guided tours 1pm, 2.30pm 4pm. Exhibitions of the Schloss Neuhardenberg Foundation (not in the castle building) Tuesdays to Sundays from 10 am to 6 pm schlossneuhardenberg.de/veranstaltungen/programm