Looking for suggestions for an itinerary. Have 14 days in August...arriving and departing from Amsterdam. Would like to spend time in Belgium, the Netherlands, and the North Sea side of Denmark.
It depends so much on your personal interests and your budget. Museums? Architecture? History? Nature? Cycling? Hiking? Food and drink? Nightlife?
I don't know, which cities you plan to visit in Belgium or the Netherlands, but for the west coast of Denmark, my suggestion is to rent a car. The area is not very densely populated and access to the beach is not very good with public transportation, you will have to rely on slow rural buses. Belgium and Netherlands is probably fine with train as long as you stick to major towns.
Also, I will suggest that you go up the whole length of the west coast. The islands in the south are pictureque, and furthest North, there are a few places of interest: a giant sand dune "Råbjerg Mile" and Skagen, the northernmost tip of Denmark where the North Sea and the Kattegat meet. You could the return along the more densely populated east coast and visit the cities along the coast.