Van Gogh and Rijkmuseum

I suggest making an online reservation for the museums. We were on the Holland/Belgium tour in 2016 and our tour included the museum pass for most museum in Holland/Netherlands. the tour include the Riijks but on our free time one afternoon we made online reservation for the Van Gogh and just walked up and got in quickly.

We were there last July. We went early on separate days and were able to get in rather quickly. The biggest line was for the Anne Frank's house. You need to buy your ticket today for that one. If I recall from other posters, peak season in the Netherland is in spring when the bulbs are blooming. Have fun.

If you are buying the Museumkaart, you do not need to buy advance tickets to these two museums. However, if you want to see the Anne Frank House, you should make online reservations immediately (if there is still space available while you are there). You can make a reservation even though you don't have a Museumkaart yet--just choose the Museumkaart option when you get to the ticket selection page. Once you are in the Netherlands, it's smart to buy the Museumkaart at a less busy site, such as the Royal Palace on Dam Square, rather than at these popular museums. Good luck.

first weekend in September which should alleviate some of the crowds

Amsterdam doesn't really have an off-season anymore; especially on the weekends. It's party central all year round thanks to the discount airlines, and high-speed trains.

The Van Gogh museum has lines at all times except late in the evenings outside weekends. Reservations are certainly advised!

I would suggest reserving tickets ahead of time for the Van Gogh and try to reserve entry as early as possible in the day. We were at Van Gogh in June 2016. If I remember entry to the museum was timed. Also any backpacks will need to be checked.