One extra, non Netherlands stop suggestions

There are so many wonderful towns and cities to visit in The Netherlands, easy by train. No interest? Leiden, The Hague, Delft, Haarlem, Rotterdam?
If you want Spain, check where flights from AMS fly to in Spain as a starting point.
English is spoken well in most of Scandinavia.

Too many good choices. Plenty nearby in the Netherlands (mentioned above), also lots nearby in Belgium: Bruges, Ghent. Forget flying anywhere. Easy.

Belgians (esp. in Flanders) may speak better American-style English than any other country on the Continent, if that's a factor for you, which you were hinting at in your question. I was recently in Bruges and Ghent, which IMO would rank over Brussels in terms of art, at least art that is in venues other than large art museums.

Your "English or Spanish with some minimal knowledge of the local language will cut it"--this suggests that Brussels would not be a contender since more French is spoken there, although English works there as well.

Amsterdam to Bruges or Ghent by train is about a couple of hours by train, and once in Bruges or Ghent, the other town is only a 30-minute train ride away.

No way I'd want to insert two additional flight days into an 8-night trip. I'd either stay in the Netherland or check out Ghent and Bruges.

However, there's an easy way to see what other cities have non-stop flights from Amsterdam: Go to the airport's Wikipedia page and look at the "Airlines and Destinations" chart. I think you'll need to skip over the seasonal flights, because they probably won't be operating in March.

Similar information (theoretically should be identical) is available at FlightsFrom.com.

If I were going to fly somewhere, I'd be tempted by southern Spain for weather reasons. I see flights to Seville and Malaga. I'd definitely choose a place in or very near the airport city. I wouldn't want to waste precious vacation time getting from an airport to a bus or train station and traveling further, only to do the whole thing in reverse three days later.

As usual, you guys are awesome. Thank you.

I'll start digging through your advice.

You are smart to give yourselves a day to acclimate.
there is a lot to see in the Netherlands; most everything just a quick train ride away.
Since it is recommended to arrive 2-3 hours before a flight out of AMS, consider day trips and keep a base of operations in or near amsterdam.
We rented a house thru VRBO in Zandaam for three days before a river cruise.
There are lots of reasonable rental options, even houseboat rentals! BTW the house we rented had bicycles for use if that is of interest to you.
We also went to Bruges and Ghent for a couple of days at the end of our trip. We took the Thalys to Antwerp (just to have the experience) and the local trains from there. There are sights there that are not to be missed!
If you are a true foodie, check out Martine's Table in Amsterdam; it was a fun experience but you need reservations and the sooner the better; she only hosts eight people each evening.
Finally, depending on your flight departure from AMS before returning home, consider your last night at an airport hotel. Several are in walking distance to Schipohl - we used the Sheraton but there are other properties also.
Have fun!

I’d book a train to Paris. It’s only 3.5 hours from Amsterdam to Paris on Thalys. You can get reasonable fares too.