When I first visited this site, it was very underwhelming; I just saw flat land and lots of grass (like pretty much anywhere else in rural Holland). However, now that I am a Dutch tourguide and know more about the place, I often drive through it and people even get excited about it. To really appreciate it, you should make a visit to "De Museummolen" (museum windmill) in Schermerhorn. This is NOT in the Beemster polder, but the Schermer polder, which is only 1 kilometer away. It is therefore not part of the UNESCO site, but at the museum they show a 10 minute movie that explains the history of the Beemster (and the other lakes that were drained in the area) and you get to go inside a working windmill as well. When there is enough wind, you can see how the windmill pumps the water up. If there is no wind; you can try pumping it yourself. There are other windmills in the area as well (you cannot visit those inside since people live in them) and a drive on the dykes around the area is very nice as well because you can see the difference in height between the reclaimed land and the "regular" land.
And of course the Beemster Museum is located in the villge of Middenbeemster and the beautiful village of De Rijp is worth a visit since it is right on the border (so the village used to have a harbour on the lake and it has a history of herring fishing and whaling).
Stock up on some of the Beemster cheese at the Cono Kaasmakers store (the store at the large factory where all the locals get their Beemster cheese for a good price, you can sample the cheese as well, my favorite is the Royaal Grand Cru) and have lunch/dinner at one of the restaurants in the area (for instance 1612 Restaurant at Noordbeemster) and you'll have a nice half day trip