Three days on my own in netherlands

Delft is a wonderful little town. It's not a bad base for exploring nearby towns like The Hague and Rotterdam by train (or tram), and of course you can visit Amsterdam itself by train easily, if you don't mind the longer train trip. It should be easy to fill up three days.

Heading south to Belgium wouldn't be too hard, either, even as a long day trip if you wanted to, depending on how much else you've seen in the Netherlands.

Amsterdam is only 1 hr from Delft with 2 direct Intercity trains per hour (and more involving a change in Leiden).
The Hague (Den Haag) is a mere 15 mins, and its Mauritshuis is not to be missed if you have any interest in Dutch painting. They've got Vermeer's Girl with a Pearl Earring! Nearby is the Escher Museum. Take the train to Den Haag Centraal, from where it's a few minutes walk.
A not so well known, but very nice destination is Dordrecht. Half an hour away, with several direct trains per hour.

Use the train planner of the Dutch railways for your journeys.

Thank you, for the suggestions. I have read the book The Pearl Earring and will take your suggestions and visit the museum. We also hope to rent bikes and tour around a bit that way as well. I am interested in the windmills and seeing the history and culture. Can you recommend a walking tour of Delft?

Thank you, for the suggestions. I have read the book The Pearl Earring and will take your suggestions and visit the museum. We also hope to rent bikes and tour around a bit that way as well. I am interested in the windmills and seeing the history and culture. Can you recommend a walking tour of Delft?

I don't see the need of a walking tour of Delft. Get a map and explore. The city centre is very compact.
For windmills go to nearby Schiedam (less than 10 mins on the train). They've got some of the world's tallest windmills, used for the traditional gin industry. They even have got a museum dedicated to this liquid pleasure (Jenevermuseum). Go there late afternoon! The historical centre is small, but very nice.
And I forgot my dear adopted city of Leiden (20 mins from Delft). How stupid! The largest historical centre after Amsterdam. Everybody loves it. Me too. Leiden has also a huge windmill, like those in Schiedam. Three minutes from the station on the way to the centre. You can visit it (if you don't mind a lot of very, very steep stairs and ladders).

Sounds great. I am also interested in going to the National Park De Biedbosch. My thought is we can take the train on Friday from Delft to the Park and spend the morning there and in the afternoon train back to Dordrecht and spend afternoon and train back to Delft in the evening. Does that seem reasonable to do in a day? Thanks

I have no experience with excursions in the Biesbosch, but if you can be in Dordrecht late afternoon, you've got enough time to wander around, have dinner and be back in Delft at a most reasonable time.
Another interesing thing to do in combination with Dordrecht is the famous windmill complex of Kinderdijk. There is a ferry (the Waterbus) leaving from the northern point of Dordrecht (Merwekade) making that a very nice trip. You can go back to Delft by taking the same Waterbus to Rotterdam (near the Erasmusbrug), from where it's a 20- to 30-minute walk to Rotterdam Centraal station. Their website has the schedule (you need Waterbus #202).

The best way to get to the Biesbosch National Park is by boat from Dordrecht. So first take the train to Dordrecht, then walk through the old city centre to "Dordrecht Merwekade", where you can take the waterbus to Dordrecht Hollandse Biesbosch.
The schedule can be found here:
https://www.waterbus.nl/reisplanner

Another way to reach the Biesbosch is to rent bicycles at Dordrecht central station. The Biesbosch is about 7 km from the station. In Dordrecht, Villa Augustus is a wonderful place for coffee or lunch.

Have you also read The Goldfinch”? The painting is also at the Mauritshuis museum in the Hague. The painting is spectacular and the book is excellent. We also enjoyed visiting Leiden.

I have now put both the Goldfinch on my reading list as well as Leiden for a possible visit. Thanks to all of you I am really looking forward to my trip and must now put a plan together. I appreciate all your suggestions.

If you are looking for books about the Netherlands, you might want to dig up "Amsterdam: A History of the World's Most Liberal City" by Russell Shorto. I enjoyed it and really learned a lot. It may make you want to spend even more time in Amsterdam.