A warm sea to swim in? Haha, you do know we're just on the other side of the same body of water, right? Our sea is surely as cold as yours, seeing how it's the same one.
But, now a more serious answer.
Regions:
The Wadden Islands spring to mind. They are a very special part of the country. You've got the sea, opportunities for water sports and beautiful nature. There's quite a lot to do for visitors. Just keep in mind that not all islands allow cars.
The south of the Country, (Province of Zuid-Limburg) is completely different from the rest of the country. We got some hills here, beautiful landscapes, the best food-/dining culture. It feels like you're in Belgium or France. There's quiet a lot of campings on farms with good views. Mountainbiking, roadbiking, caves and of course the lovely city of Maastricht.
In the center of the country we've got National Parks like 'Utrechtse Heuvelrug' and 'Hoge Veluwe'.
Good for walking, biking. Utrecht is in the centre of the country and worth a visit (canals, old buildings, lots of history).
Amsterdam, Rotterdam, Den Haag are all withing an hour drive from Utrecht. These are our bigger cities. The area is called Randstad. Leiden en Delft are lovely, smaller towns.
We have quite a long coastline. Scheveningen is the most touristy/busy. If you prefer a quieter beach you can try Kijkduin in Den Haag. Bloemendaal is famous for it's beach bars. There's Katwijk aan Zee and many, many more when you drive north.
Camping:
Always hard to recommend camp sites, because it's such a subjective thing and also depends on which time of year you stay.
If small green sites are your thing then try
https://www.natuurkampeerterreinen.nl/?lang=en They have an English version of their website. They focus on small, green sites. Most other sites you'll find will be bigger and have more entertainment, but we do not have many large holiday-village style sites. Sites in the North and south of the country will generally be more beautiful/in nature, but it will differ per site.
If you have any other questions or want details, just ask.