transportation needs from Ghent, Belgium to the Kroller Muller Museum

I’m not sure feasible this is - it would take about 2.5 - 3 hours to drive from Gent to Otterlo, where the museum is. You could probably get a taxi to take you from the museum to the nearest train station ((Apeldoorn? Arnhem?) but then it would be at least an hour’s journey to Amsterdam, and longer if you drove all the way. If your flight home is later the next day, you could perhaps take a taxi to Arnhem, stay overnight and then get an early train straight through to Schiphol if you’re prepared to risk it.

if you did rent a car in Belgium - I'm not advocating that, just exploring the possibility - and return it in the Netherlands you will have to pay a very high drop charge.

Traffic around Antwerp is awful day and night, and the tunnels have a toll.

To avoid the drop charge you can take the train from Ghent to Breda (about 2 hours) in the Netherlands and rent a car there for driving in about 1½ hour to the Kröller-Müller Museum near Otterlo.

The Kroller-Muller museum is splendid, and has some Van Gogh and James Ensor works that have been shown elsewhere, but not often. And on a nice day, the surrounding park with free basic large-tire bicycles is very nice. But as world-class museums go, it is small. I would recommend that you spend the day in Antwerp museums, or in Den Haag museums, and then return to Amsterdam. If the bike tour does not include Brussels, the unified old and modern art museum there is a better use of your time, almost impossible to carefully view in just 8 hours.

The main thing that you should be worried about is not car rental but the fact that you plan to do this on April 27. April 27 is Kingsday, our national holiday. It’s celebrated everywhere in our country, but the festivities in Amsterdam always attract huge crowds. Depending on where you plan to stay in Amsterdam, it may be very difficult or even impossible to reach your accommodation in the afternoon/evening because of the crowds. Busses and trams in central Amsterdam won’t run at all on that day.

What does your exact schedule for that day look like? What time does your bike tour finish? Where do you plan to stay in Amsterdam?

Our flight home to USA is Sunday morning. We are hoping to book a hotel near the airport to basically spend the night.
We were looking for something interesting to do from noon Saturday till the evening, not In Amsterdam. Yes we are aware of King's Day from our friends who lived in Amsterdam for many years!

We love museums and will have visited the Van Gogh and Rijks on the front end of our trip. We are open to suggestions of interesting places to go between Ghent and Amsterdam for the afternoon. Had originally thought to drive to Kroller Muller would be the easiest and less time than train or bus but also seeing they are only open until 5 pm and wondering if that might not give us adequate time to enjoy. appreciate the suggestions!

You don’t mention what places you will have already visited on your bike tour, so it makes it a bit difficult to help you.
Antwerp could be an ideal stop, plenty of very interesting museums and you wil need to switch trains there anyway on your way to Amsterdam Schiphol Airport. Store your luggage in a locker and walk into the city. A few weeks ago I finally had the opportunity to visit the newly renovated KMSKA, the Royal Museum of Fine Arts Antwerp. I highly recommend this museum!
The Red Star Line museum is well worth a visit too.

Even though I love the Krolller-Moller museum very much, I don’t think it’s an ideal place to visit on your way from Ghent to Amsterdam.