Here's one that I would skip, if I had a tight itinerary. Waterloo (as in Belgium, where Napoleon was defeated.) It's just a hill. That's it. Unless being able to take pictures and say you were there is what floats your boat, there's really nothing much to see.
Here is one not to be missed, if you're in the Netherlands. The Kroller- Muller Museum, which is in the National Park the Hoge Veluwe. Absolutely amazing. It's out in the middle of nowhere, nestled in the forest. When you enter the park/forest, there are these huge banks of "free" bikes that anyone can pick up and ride around the park. Whenever you get tired of riding, you can drop the bike off at another bike parking area. Then you can walk around the forest and see an enormous collection of outdoor sculptures. And when you get tired of that, you can go into the museum itself and see the most enormous collection of Van Gogh paintings. Definitely worth the trip out of Amsterdam.
And here's one closer to home for a lot of us, and free, other than whatever it costs to get you there. The National Mall when the cherry blossoms are in bloom. O M G. It is beautiful. The ex and I made this trip on the spur of the moment, one sunny Sunday years ago. It is so beautiful. You have to be ready to go, when the cherry blossoms are ready, though.
Another thing not to miss. Drive-in movie theaters. I've visited one in every place I've lived (even though, sometimes, it was quite a drive.) It's a unique experience. This site has a listing of functional drive-ins.