ATTENTION: For years I've been promoting the Museumcard. However, in 2018 they changed the rules making it far less attractive AND complicated for foreign guests. Now the temporary Museumcard you purchase at a museum allows only five museum visits. Until you register, and the official card is send to you. Will be within five working days they claim.
So in any case you need an address in the Netherlands (or Belgium they say). Once you received the official card by mail your temporary one is not valid any more.
So I'd say, from now on only purchase a Museumcard if you're staying more than a week in the Netherlands, and know for sure where you will be staying. Or make an arrangement with friends or relations in Holland who can purchase it for you in advance.
The Museumcard remains of course a great tool for visiting.
If you're planning to visit more than a few museums in the Netherlands, it pays to buy a Museumcard. It gives free entrance to over 400 museums throughout the country, including all major ones in Amsterdam.
For most museums tickets for adults range EUR 8,50 to 15,- .
A Museumcard for adults costs about 65 Euros (in 2019). Under 19 years half price. Valid one year.
Plus, there's another great advantage : you can go to the same museum more than once. Especially convenient for the large museums, for coming back a second time with more focus.
And don't forget that multitude of charming small museums you're bound to stumble upon when visiting. With a Museumcard you'll sooner hop in, and have a look. That's just how it works.
So in all respects, a Museumcard helps you being flexible with your visit to the Netherlands. And with Dutch climate in mind, that too is something to keep in mind. See page When to visit what.
A Museumcard can be bought in museums (not in the very small ones). After you filled in a form, and paid, you can use the card immediately. With restrictions as of 2018 - see above.
On the Museumcard website www.museumkaart.nl (in Dutch only), there's a museum list.