Direct regional-express trains leave every hour leave Düsseldorf's main train station (Hbf, short for "Hauptbahnhof") for the city of Münster. The trip takes 90 minutes. I have spent half a day there myself but wished I had spent longer.
Münster has what you are looking for. Rick's materials for Germany are somewhat limited to places that already have strong name-recognition for his mostly-North-American audience; Münster is one of many "back-door" options that have been left out, so you will need additional resources for planning. As usual, the local tourist office website has very complete information:
https://www.stadt-muenster.de/en/tourismus/service-and-information
https://www.tripadvisor.com/Attractions-g187382-Activities-oa0-Muenster_North_Rhine_Westphalia.html
The cost per person for the round trip, when done by day pass - 2 day passes with 4 persons on each pass, in your case - comes to roughly €14. Just buy this day pass at the station prior to boarding.
SchönerTagTicket NRW 5 Personen
https://www.vrr.de/en/tickets-fares/ticket-overview/ticket/nrw/schoenertagticket-nrw-fuer-5-personen-day-pass-5-persons/
Train schedules:
https://int.bahn.de/en
- Use "Münster(Westf)Hbf" as your destination. Type "M-u-e-n-s" and this station will appear as a pull-down option.
- Also: to find the hourly regional express trains, be sure to select "Local Transport Only" under "Mode of Transport" when you do your search.