I agree about taking them to a milonga. You can also find out which milongas feature an exhibition by a guest couple at around half-time - so they can see authentic social tango, and a performance geared not for tourists but for tango people. If you like the milonga idea, with our without a 3-4 piece exhibition interval, the next step would be to choose the milonga. The dancing and the atmospheres vary widely. You can get some suggestions from forum people here, or ask your teachers for recommendations. I think a number of us agree that the milonga solution will be a more satisfying experience for them. And as Jantango says, "the price is right".
If your guests like the experience, you can give them this url for finding tango in Amsterdam and the rest of the Netherlands: http://www.torito.nl/agenda/