Okay, so here's some more ideas! I'm no good on fixed-base camping, as we've always done touring and hostels. But here's some places my offspring have enjoyed as young teens:
- From Calais, you could do worse than just staying on the Belgian coast. Lovely cycling along the coast (some nice big public art I recall), nice cycling along the canals.
We particularly enjoyed the hostel in Oostduinkerke, called De Peerdevisser. If you head out to the beach first thing, you might see the horses - they are used for shrimp fishing (hence "peerdevisser").
Nearer the border with the Netherlands, this place absolutely worth a visit:
https://bizarium.com/en/Ghent, Brugge: very nice, possibly a bit busy. We've done them with the kids on tandems, I think I would avoid now that we are all on solos (or stay outside the town and public transport in). Bruges does have a great chip museum though.
Mechelen is a nice little city. If you are into science museums then this place is great:
https://www.technopolis.be/en/- To add to Yoav's suggestion of Rotterdam/Hook/The Hague, here's some things we enjoyed round there:
the dune cyclepath from Hook to Scheveningen (ideally with the wind!)
stay in the cube houses in Rotterdam:
https://www.stayokay.com/en/hostel/rotterdam(worth also visiting the museum cube house)
ESA Space centre at Katwijk:
https://www.space-expo.nl/enEscher museum in The Hague:
https://www.escherinhetpaleis.nl/?lang=enWe enjoyed Zaanse Schans more than Kinderdijk:
https://www.dezaanseschans.nl/en/All the windmills are different, milling different things. We found Kinderdijk a bit too busy for our liking last year. Whereas Zaanze Schans is a bit more spread out and really interesting - I remember being wowed by the sawmill and a paint mill.
I reckon at 13 this place might be a bit young, but I've included it anyway as yesterday my 15 year-old expressed an interest in returning:
https://www.linnaeushof.nl/en/So it's like a funfair theme park, but everything is people-powered! Great fun.
- the Limburg area of Belgium is lovely. Close enough to the Netherlands to enjoy trying another country, with the previously-mentioned points of interest (cycling through trees is huge fun, we spent an hour last year going round and round) and more besides. Enough tourist action to make things easy but not that busy. The Belgian canals are great for cycling along.
My kids have enjoyed staying in hostels because of the wonder that is sprinkles (Hagelslag) for breakfast.
The only snag might be booking somewhere - you might find that there's more availability towards the end of August than the beginning of the summer holidays, depending on how touristy the area is. But basically, I reckon you can't really go wrong!
