Question about itinerary and Sunday closures

You would indeed have trouble to find a museum with a Sunday closing.
But I don't understand the geographics of your itinerary. I understand you want to visit Den Haag on your return trip from Gent to Amsterdam. The next day you'll pass the same Den Haag on your planned day trip to Leiden and Delft. Combining that day trip with Kinderdijk, if I understand correctly, is not very well possible. It will take you around 1 hr to reach from Delft.
How do you plan to combine bicycling in Waterland with visiting Hoorn and Enkhuizen? You must go back to Amsterdam to return your bicycle, or otherwise go to Purmerend where you'll find a railway station to get you to Hoorn and Enkhuizen, and either leave your bicycle there to pick it up on the return journey or take it with you on the train (in which case you shouldn't forget to buy a bicycle ticket for EUR 6,20).
You could combine your day trip to Utrecht with visiting Naarden, a perfectly preserved 17th century fortified town and probably more of interest to you than the Muiderslot (which nowadays is promoted as »Amsterdam Castle«). It is a 20-minute walk from the station Naarden-Bussum, from where you'll find direct trains to Utrecht.

Andrew, my layover is set and I can't change it - and I would like to see Iceland, even if I only get a day and a half in real time. :) I must not have read the info about Frans Hal correctly - but I would very much like to see Grote Kerk and Carrie Ten Boom. But maybe I could split the trip up in two. Or possibly stop in Harlaam on my way back from Gent, instead of Den Haag.

Ton, thank you so much for this - obviously, I haven't thought this out very well. Would it be possible to take the train up to Hoorn and/or Enkuizen, visit up there and then head back down to Marken, rent a bike there, and travel back to AMS? Or even take the train back down to AMS, and then rent a bike for the second part of the day? Or even an evening bike ride? I'm having a hard time wrapping my head around the logistics of this part.

Naarden sounds wonderful and I am sure I would enjoy it much more than Amsterdam Castle - thank you for the recommendation!
And I get what you are saying about the trip to Den Haag, etc. I was worried that I would not have enough time to see Den Haag, Delft and Leiden in one day, which is why I thought I might split it up. Do you have any suggestions? Or should I just cut out Den Haag? I am interested in it because I am an attorney and I like MC Escher. But Den Haag is not a deal breaker - I really want to see Delft and Leiden, though. What about adding Kinderjijk on my way back from Gent? It looks like I could stop in Rotterdam and take the bus there.

Some suggestions:
- Skip cycling in Waterland (Broek, Marken) and limit yourself to Enkhuizen and Hoorn. Both lovely towns (Enkhuizen is my favourite) which will keep you busy for the whole day, certainly when you include the open-air museum (Zuiderzeemuseum) in Enkhuizen. In Hoorn the Westfries Museum is very well worth visiting. You won't have problems finding a place for lunch and/or dinner in both places. In Hoorn the square in front of the museum is a good place (weather permitting of course). Have this as a day trip at your first stay in Amsterdam.
- When returning to Amsterdam from Gent, stay in the city using the afternoon to do the things you've missed at your first stay.
- Combine Den Haag and Leiden. I can't advise you on the order, not knowing when you can visit the International Court of Justice (if at all). The best place for having your lunch or dinner in Den Haag is on the Plein, in the very heart of the city, less than 10 mins walk from Den Haag Centraal station. In Leiden you'll find good places for lunch at Van der Werff (3 mins from the station, you'll pass it on the way to the centre) or Voorafentoe (on the Nieuwe Rijn canal behind the city hall).
- Combine Delft and Kinderdijk. Delft is a small city, which can easily be seen in a few hours without hurrying yourself. In Delft you can take the train to Rotterdam Lombardijen (possibly a direct all-station Sprinter service, saving you a change at Rotterdam Centraal) to pick up the bus for Kinderdijk (schedule on 9292.nl; the bus stop is at the right side of the Spinozaweg on leaving the station as shown on this map). After having enjoyed Kinderdijk you must take the Waterbus 202 to get the real experience of this part of the country. Either go back to Rotterdam or - strongly advised - continue to Dordrecht, which is a lovely city (why not have your dinner there?). Train back to Amsterdam goes every 15 mins. This is the schedule of Waterbus 202.

When you are in den Haag you are just a hop skip and a jump from Delft. I know what you want to see in den Haag but I don't know what you want to see in Delft so it' s hard to advise how long it will take you.

The Mauritshuis has fabulous paintings (be sure that the one you want to see hasn't been sent out on loan somewhere). The Palais which houses the Escher was pretty good when I was there. I enjoyed my visit and I am an Escher fan too. But I wasn't there for too very long as I remember. I think it has been improved a lot, with the interactive upper floor it is probably now a longer experience.

To add that, and what you may want to see in Delft and what you may want to see in Leiden may make a pretty long day. At least all three cities are compact.

Naarden is very interesting and with it pretty much as it was.

Ton and Nigel, thank you both for such informative responses - that is very very helpful! And it makes much more sense than my itinerary. I am amazed at the knowledge and generosity of posters on this board. I should also mention that I don't have anything in particular to see in Delft - I'm not a fan of Delft pottery, either - but many posters here had talked about how charming the town is so I thought it might be worth a visit.

Andrew, yes, I probably should have explained that. WOW Airlines is just starting to fly out of CVG and it includes a layover in Iceland, which you can stretch out as long as you want. I have always wanted to see Iceland so I extended my trip by a few days so I can spend a bit of time there. But the main focus of this trip is the Netherlands.

Delft has a nice central square, even nicer in my opinion is when the market is not set up. Many people here love to visit and wander European markets , strolling from stall to stall with a little grazing. That's great and has been part of the tradition for hundreds of years. Personally, in a small city with as beautiful a centre as has Delft I prefer to see the architecture not the canvas of the stalls.

But different strokes for different folks.

There are also two churches to poke around in (at least), one grey and one brick. Nice views and shops around the centre, and we used to enjoy pancakes on a small houseboat moored near the TI. Don't know if it still there - it has be a few years since we have been back to Delft.

If you don't have specific things you want to see there, and if you will have had several other beautiful small cities like Bruges, Gent, Leiden and Haarlem, as well as the bigger yet still beautiful places like den Haag and Amsterdam and Utrecht perhaps you will decide how much time to give Delft.

Mardee,
I just returned from Amsterdam/Haarlem, 6 days there.

From Haarlem, I took the train to Delft, spent 3 hours or so there, delightful town, New Kerk, Old Kerk, great for wandering. Then, about 1:30, walked back to the train station and hopped on for the den Haag. All I really had time for was the Mauritshuis which is magnificent! Some wandering here, too. I never once stopped for lunch or a coffee because I needed to be back in Haarlem for dinner, around 6 pm. It was my birthday and I wanted to eat in an Indonesian restaurant. I made it and enjoyed a wonderful dinner. Then I walked back to my hotel and packed. My flight home was the next day.
My point here in recounting my day is that you may be too ambitious in wanting to see 3 towns. It takes time to get to a place, then walking from the train, etc. eats up a lot of time.

I haven't checked, but I assume you'll get yourself an OV Chipkaart. It costs EUR 7,50, but saves you EUR 1 each time you buy a separate ticket. It needs a balance of at least EUR 20 to use it on the train, but - with, I expect, a little bit of paperwork - you can get that refunded at the end of the trip. (It does not work in Belgium, one of the deplorable results of their misguided independence in 1830.) You can buy it at the window of any larger railway station.
And the same is for the Museumkaart. Costs you EUR 59,90 but gives free access to almost all the places you want to visit. (Not, I'm afraid, the Escher Museum, and the churches in Delft, mentioned above.) You can get it al almost all participating museums. (List is arranged by province. For The Hague, Delft and Leiden you have to look under »Zuid-Holland«.)

"but saves you EUR 1 each time you buy a separate ticket."
To save that euro there is no longer a need to buy an anonymous chipcard. NS has finally entered the 21st century. Nowadays you can buy tickets online (ns.nl/en) and pay using a credit card. These tickets do not have the 1 euro surcharge. You'll either have to print these tickets or load them onto the NS app. An advantage of e-tickets is that you dont have to touch in and out at stations, unless the station has ticket barriers (but that is only to open the barriers)

"...with, I expect, a little bit of paperwork - you can get that refunded at the end of the trip"
https://www.ov-chipkaart.nl/terminate-or-replace/refund-of-credit.htm

Thanks, Judy - I think you're right about overdoing it. I like to see a lot but not at the expense of whizzing by so fast you can't really see it. :)

Nigel, that's a good plan - I do have specific things I want to see in the other places so I think I will play it by ear. If I have extra time, I may head over to Delft and will look up the places you mentioned (the churches sound nice - I have a "thing" about seeing churches, which is ironic, since I'm not at all religious. :) Otherwise, I will enjoy all the other places.

Ton, I had already planned on getting the Museumkaart, and in fact, have already reserved my tickets online for the Anne Frank House and the Rijksmuseum. I'm still debating whether I should get the OV-chipkaart or just get the GVB 4-day ticket (at least for the first half of my trip). I know you have to check in and out, but it might be worth it to avoid having to deal with the refund.

Floris, thanks for the info - as I mentioned above, I may just get the 4-day ticket and deal with checking in and out. But if I did purchase the OV-chipkaart, it looks like you can only buy it online if you are a resident of the NL, Belgium, Germany or Luxembourg. Am I missing something?

Thanks to all of you - I really do appreciate the time you spend in helping other travelers!