From the 9th til the 22nd August you can buy a day ticket which covers train travel through the whole of NL. That is a real bargain as they normally sell for €45,40.
Good thing about this offer is that you can buy then singly so great if you are travelling alone.
Some T&C's as described in the link below but only in Dutch. Basically you buy a code at the till in the supermarket and this needs to be activated on the website. You then need to print off the ticket so you will need internet access and a printer I guess.
Travel also needs to be off-peak, so after 0900 weekdays and the whole weekend.