There is a daily free train travel card that costs € 50,80 for second class travel on NS trains (so it leaves out a couple regional railways in the outer areas). It allows unlimited use of trains for one calendar day (not for 24h, notice the difference). It comes as a disposable OV-Chipkaart (an RFID card that you use to check-in and -out at the totems located on train stations, even with a day pass you must check-in and -out except when transferring from one train to another). More info here:
You can buy several day passes from the machines that take credit cards at Schiphol airport, if you want to. Just never check-in with several cards on your purse, or else you might risk simultaneously activating several of the cards at once and then, obviously, losing them.
As for the Dal Voordel (40% off-peak + weekend discount), there is a catch: you need to have a personal OV-Chipkaart and it takes a couple days, and a Dutch bank account, to get one.
Sometimes big stores or chains have promotions where they sell discounted day passes.