Day Trip to Kinderdijk/Delft from Haarlem

That's indeed the best route to Kinderdijk. Here is the schedule for the direct ferry, which starts from 1 May. In my memory it's somewhat longer from Rotterdam Centraal to the Erasmusbrug.
After visiting Kinderdijk you can continue with the same Waterbus #202 to Dordrecht. A much overlooked historical city. The arrival from the water is wonderful. From Dordrecht you have frequent trains back to Rotterdam and Haarlem. Combined with Dordrecht you've got your day well filled.
Delft can best be combined with The Hague (Mauritshuis, Escher Museum, Panorama Mesdag). Or, for something different, Schiedam, which has a very nice historical centre and, if you're into that, a museum dedicated to the industry which made this city famous: gin.
I don't see the value of a boat trip at Kinderdijk. It has foot and cycle paths going along the windmills. This OpenStreetMap shows them as blue and red dotted lines.

Thanks so much, tonfromleiden! I think I read that tickets can be purchased on the waterbus. Is that right?

Also just read that May 5th is Liberation Day. In planning itinerary, can you tell me what will be closed on that day. The 4th looks like silent prayers during day, but on 5th there are celebrations. What should I expect? Thanks again for your help in my planning.

You can buy the tickets on the ferry.
May 4th is Remembrance Day. You won't notice much of it, except for a few minutes' silence at 20:00 and shops closing early. May 5th is Liberation Day. It is an official holiday only every 5 years, the next one in 2020. Most things will be running normally. (I always forget these holidays, so I had to look it up.)

Thanks again tonfromleiden!! You're wonderful!!

And now hoping for good weather!

:-) Definitely!! I'll probably have more questions as I continue to plan. Stay tuned.

The Waterbus also accepts the OV-Chipkaart to pay for fares, and fares are cheaper this way.

I think you might be confusing the small boat cruise within Kinderdijk and the boat that takes you from Rotterdam. The latter goes through a busy waterway. The cruise boat within Kinderdijk is an optional activity you take within the windmill park. It is neat, although you can just walk alongside the raised levee all the way through the windmills. This cruise uses a small boat in a tranquil canal that is physically isolated from the main waterway and some 5-6 meters below the water level of the Lek (the Maas outlet that gives access to Rotterdam from there). In other words: the mode of transportation you choose from Rotterdam to Kinderdijk will not affect the way you see the windmills themselves.

Thanks, Andre L. That helps in understanding my trip to see the windmills. Also need to look at transportation options in general. We'll be in Haarlem 5 nights and Amsterdam 4 nights and will travel from a/p to Haarlem on day 1 and then day trips 3 add'l days from Haarlem plus 3 days while in Amsterdam. I will look more closely at the OV Chipkaart. Will travel to Keukendof, Delf, Rotterdam, Alkmaar, Horn, Edam, Markham---some of which will be done by bus. Any help anyone can give on this would be appreciated.

The OV Chipkaart is practical. Check in and check out on all kinds of public transport without the need of buying tickets. The downside is that the card costs EUR 7,50 without reimbursement and it needs to be loaded with a minimum of EUR 20 to be accepted on the train. I have no idea how you can get that money back. Maybe Andre can help you.
Some destinations on your list might be less enjoyable than you imagine. The Keukenhof will be crowded, very crowded. Marken is touristy, very touristy.
Alkmaar and Hoorn can be combined. Take your time to enjoy these cities. The Westfries Museum in Hoorn is well worth a visit.
And have another day trip to Delft and The Hague (Den Haag).

Thanks tonfromleiden. On an earlier post read about the TripKey which is relatively new but seems best option created for tourists. It charges credit card for each trip for a small 3.50 euro one time fee and offers same discounts as card your recommended. Pick up after arriving in Netherlands at various hotels or a/p when it is activated. Having one card to swipe w/o having to add money will be convenient. Thanks for responding. Still in trip planning mode so more to come. Appreciate your responses.

I have no experience with the Tripkey, so I was hesitant to mention it. This is their website, including a list of pick-up points. Local tourist offices seem to offer it also, which gives me some confidence it works. Good luck! And have fun!

If you have less than € 30 on your OV-Chipkaart, you can get a refund in cash for the balance paying a € 2.50 fee only.

Thanks so much for responses. The Trip key seems to be easiest card.