will be arriving in Amsterdam via cruise liner in April..We would like to spend 3-4 days in Amsterdam and then move south. We will fly out of Frankfurt 2-3 weeks after arriving in Amsterdam. Suggestions as to things to do assuming we intend to stay at each location at least 3 nights...thanks...
With 2 weeks to kill between Amsterdam and Frankfurt, you choices are wide open. Don't think you have to stay around the Rhine Valley. You could zip down to Paris in a heart beat. Cologne is nice, as is the Mosel Valley and Rhine gorge area. Just look at northern France, Belgium, Germany and pick your spots.
I would second Cologne and add Liege, Tubingen, and Ulm, then to Franfurt. Mainz too.
I would be useful to know if you like art museums and whether you'll be training or driving. Also, are you interested in outdoorsy and bicycling, or more urban tourism? All these things are possible - there's more here than you can see in 2-3 weeks. Is fine food important? (I found the Netherlands to be a disappointment in food and hotels, not in attractions.) Here's one itinerary, of many: Amsterdam 4 days, Den Haag/Rotterdam/Delft/Kinderdijk 3 days, Antwerp 5-6 days including Brussels (1 or 2 daytrips), Gent, Bruges (splurge on a hotel in Antwerp), Aachen/Cologne 3 days, Koblenz or one of Rick's medieval towns 2 days, Mosel 3 days, Frankfurt. This trip skips Northern Germany, but it would be a shame to miss Eastern Belgium. I left out the medieval towns and Open-Air museum in Western Belgium. I also neglected easterly Netherlands, but only the Kröller-Müller Museum in Hoge Veluwe National Park is a must. There is an important Art Fair in Maastricht but it will be over before your arrival.
I drive the route from Frankfurt to eastern Belgium and back at least once a month, on both sides of the Rhine, and I've found all sorts of interesting stuff along the way... but before I get started, tell us your mode of transportation, as that will make a big difference in things I could recommend.