The Dutch railways offer a standard day-ticket (Dagkaart). This gives you unlimited travel on the Dutch rail network for one day. It costs around €50 and is worth considering if you are making longer trips. For example, an excursion such as Amsterdam to Maastricht, to Arnhem and then back to Amsterdam would be around €20 cheaper with the day-ticket.
Periodically, there are much cheaper options! Dutch stores such as Kruidvat, Blokker, Albert Heijn, Etos & Hema offer similar tickets at prices in the region of €10 – €20. Tickets can be purchased in store up or online and usually are restricted to travel on weekends or outside of rush-hour periods.
The local website Treinreiziger has up-to-date information on these offers.