Teenager in Europe for first time- Beginning in Amsterdam and ending in Paris... where else to go?

From Amsterdam go to visit Kinderdijk (Windmills) as a day trip.

Just like anybody else -- what interests the travelers ?? Is the 19-year-old excited about the trip, or just "whatever"? Does Dad have special places he wants to share with her?

Berlin indeed is quite a trendy place for adolescents. If you want to go there, consider to take the high-speed "ICE" train from either Amsterdam or Utrecht to Berlin, instead of flying. It will take a bit longer, but this will probably be a new experience, and you can actually see the landscape along the way.

You ask for suggestions for must-sees-and-dos " keeping in mind the desires of a 19 year old". Young women of that age are so different, one from the other, that you really need to tell us what her desires are. Has she expressed any particular excitement about one place or the other? Are Berlin and Saltzberg mentioned by you, on her wish list? How about art and architecture and history? Concerts? Shopping? Anyway, you get the idea. Elucidate, please.

A 19 year old is not a child but is at an age where she might not be the most fun traveler (depends on the kid of course) It is critical for her to have agency in her own travels. The person to be planning this trip is the daughter with input and collaboration from her Dad who is also traveling. With three weeks I would probably think in terms of a couple of anchors for longer stays and a few shorter stops.

For us, Amsterdam is the perfect 3, maybe 4 nights town. Berlin, Prague, Vienna are all possibilities and of course London would be easy as well. Or fly from Amsterdam to Rome and visit, Florence Venice and Rome with a sidetrip to Siena and then fly to Paris for the last week to 10 days. The big thing is to go where the travelers want to go and to not do too many quick stops. Any large stop deserves 4 nights or so/3 full days. The occasional 2 or 3 night stop is fine -- but shouldn't IMHO be the rule. We used to do something like fly into Rome and spend a week then spend a week on the road with several 2 night or occasionally even one night in transit stops, ending in Paris for a week. e.g. once traveling with our teen daughter we spent a week in Rome, drove to the Cinque Terre for two nights, drove to Aix en Provence for two nights, drove to Semur en Auxois for a night and then finished with 10 days in Paris.

Just as an indication of what is possible: Amsterdam-Berlin-Prague-Vienna-Venice-Turin-Paris would give a taste of a large chunk of Europe, and could nicely and comfortably be done by train, giving an impression of the land in between as well.

I know you mean well, but it's your husband and daughter who should be doing this planning. You could be a great help with the logistics.

They both need to explore the options and separately come up with their own must sees and dos. Then they can compare them and find out if they have chosen some of the same things. If so, that will make the planning much easier.

Not knowing anything about their interests, it's just about impossible to recommend anything.

Three weeks is not very long. Based on several trips and quite a bit of time in France, Belgium and the Netherlands, they could easily use all that time in northern France, Belgium and Amsterdam and still not see everything.

I can also see an Amsterdam-->Berlin-->Paris big city triangle trip using the train. I love going on the train. I can travel that way for hours just gazing out the window, but if they're "are we there yet" types, they might be bored to tears doing that.

It's about 6 1/2 hours from Amsterdam to Berlin, whether you have to change trains 0, 1 or 2 times. I'd always pick the 0 change option. It's about 8 1/2 hours from Berlin to Paris, whether there are 1 or 2 changes. The 1 change options seem to go through Nuremberg or Brussels. The 2 change ones seem to go through or near other cities. So they could choose their preferred route, maybe even spend 1 or 2 nights somewhere along the way. Having lived in Nuremberg in the early 80's, I'd naturally pick that amazing city.

I wouldn't add Salzburg into the mix unless they have some burning attraction to it. So far I haven't felt that burn.

Whatever they decide, they need to get busy making those decisions. The summer is coming very soon and lodging is definitely filling up. If they end up with at least 4 nights in any city, an apartment might be a good option. If hotels or B&Bs are preferred, they'll need rooms with two beds of some type. Booking.com lists all kinds of accommodations, including apartments. Homeaway/VRBO is a good source for apartments. A lot of people like AirBnB. I'm not one of them.

A 19-year-old adult female is likely to have just as much of a variety of interests as any other age of adult.

My now-18yo daughter chose to visit Berlin on a trip we did almost two years ago, but Paris is still probably her favorite European destination. Berlin, I have read, has a great scene for young people, but is your husband's daughter likely to do that alone, or with her dad? My daughter likes art and museums, and doesn't really like to shop (same as her parents). What has your husband's daughter liked to do on vacations in the U.S.? Does she like to go to big cities and eat gourmet food and go to museums, or go to a beach destination and relax, or go to the mountains and hike? She's likely to enjoy the same categories of things while vacationing in Europe.