@nprofile1q... not living in .nl for years anymore, thus not close there; and hopping down here would take you too long, but:
When in Gouda go get _fresh & warm_ stroopwafels from the real original place:
https://bergsbakery.nl and go pickup a Vlaai at
https://www.multivlaai.nl/multivlaai-gouda for instance a
https://www.multivlaai.nl/abrikozenvlaai (oh enter something random like '2804KK' to bypass their silly zipcode question). and yes they sell them per piece and can ask 'fill me a box with a piece of everything' ;) many things to chose from
https://www.multivlaai.nl/vlaaien but depends on what they have made that day.
My prefered: apricot (abrikozen), appelkruimel (applecrumble), kersen (cherry).
Strolling around downtown can be nice (though not been in a few years), with the castle-alike town hall, you might be lucky to catch the cheese market. If you like architecture / stained glass then
https://en.wikipedia.org/wiki/Sint_Janskerk has huge stained glass windows.
There is
https://www.museumgouda.nl though have never been there myself as it is from the last several years.
In The Hague, decent down town to walk through, many different museums, the governmental area can be interesting to walk through (quite accessible); of course there is 'the beach' aka Scheveningen (try prononouncing that properly) where you can find:
https://hetpannekoekenhuisjescheveningen.nl/poffertjes-scheveningen/ yes, poffertjes, something you need to try.
Next to all this is
https://en.wikipedia.org/wiki/Delft which might be interesting from a downtown area with the waterways there and a historical perspective ("Delfts Blauw" comes from there amongst others); also where
https://www.tudelft.nl is located.
For Rotterdam itself you will very likely enjoy:
https://maritiemmuseum.nl/enBut I would go to Amsterdam, just in case you have not seen it yet:
https://www.rijksmuseum.nl/en/stories/themes/the-night-watch sitting in front of it for a few minutes to be able to see the details is definitely worthy.
Another thing to look there is
https://techinc.nl as one of the hackerspaces. Or ask on NLNOG IRC (
https://nlnog.net/get-involved/), bet you can arrange a tour of the various datacenters with somebody if you ask ;)
Most important thing: just walk everywhere, downtown areas, typically where trains stop etc, tend to be older buildings (well, not Rotterdam that got bombed to nothing in WW2) with lots of activity and things to see and do and alive with people, many bars and pubs and parks etc. Hence, like most of Europe, having good boots and going around can be great.