There is now a "Holland Travel Ticket" for 39 euro, valid on all buses, trams, metros and trains in the Netherlands in a day, except between 0630 and 0900 weekdays. As usual with Dutch day train tickets, it is not valid on Thalys and for ICE between Amsterdam and Arnhem and Intercity Direct between Schiphol and Rotterdam a supplement is needed. A peak version is available for 59 euro.
https://www.ns.nl/producten/en/alleen-te-koop-via-station-of-kaartautomaat/p/holland-travel-ticket
For those who want trains only there are sometimes cheaper options. Various Dutch retailers often have deals giving very cheap day tickets, but they are often only for sale for a short time and often need you to visit the shop and get a voucher code that needs to be input into the NS website to print off a ticket
http://www.treinreiziger.nl/reizen/kortingsacties
The Holland Travel Ticket is always available so is more accessible to international tourists. Day tickets for train only are also for sale on the Harwich to Hoek van Holland ferry (17 euro when I bought one in May) and also at Eindhoven airport (something like 22 euro)
https://www.ns.nl/producten/en/alleen-te-koop-via-station-of-kaartautomaat/p/holland-travel-ticket
For those who want trains only there are sometimes cheaper options. Various Dutch retailers often have deals giving very cheap day tickets, but they are often only for sale for a short time and often need you to visit the shop and get a voucher code that needs to be input into the NS website to print off a ticket
http://www.treinreiziger.nl/reizen/kortingsacties
The Holland Travel Ticket is always available so is more accessible to international tourists. Day tickets for train only are also for sale on the Harwich to Hoek van Holland ferry (17 euro when I bought one in May) and also at Eindhoven airport (something like 22 euro)