This year myself and DH and our 2 DC (10 and 8) went to North Wales for 2 weeks. We hired a self catering place (and mostly self catered). We went to the beach, did some shortish walks (up to 2-3 hours), visited all the CADW Castles and National Trust places within reach (have memberships) and also did some of the touristy things such as going down a slate mine, taking a steam train ride and visited Electric Mountain.
We all really liked the "having something to do each day" and it was all interesting enough both for the DC and for the adults in the family.
We'd like to have a similar sort of holiday next year but are struggling with finding a destination that has enough to provide interest for 2 weeks without breaking the bank.
DH fancies going abroad and has suggested the Black Forest, but I'm not sure there is enough to interest the DC for 2 weeks. I am clueless and open to all suggestions ...
Provence? We stayed twice in the area south of Avignon and loved it.
Or around Toulouse and Carcassone?
Germany - you will find lots to do. I am German and we did 2 weeks at Lake Constance, walks, tourist attractions, castles, swimming (if the weather is warmer than this August). I can't remember the Black Forest but if you like mountains you could go to Bavaria. Lots of culture, lots of nature. The northern part of Bavaria, Franconia, is more hilly than mountain and you have town like Nurnberg and Wuerzburg. a lake district and lots to do for children.
Northern Germany has the Baltic Sea with lots of towns around to visit. The North Sea is also great but I find it less interesting if the weather turns bad as a lot of the area behind is marsh land.
Holland? We have spent many a happy holiday in a bungalow park called Het Grote Bos, Doorn. Google it. There is loads to do, railway museum, Amsterdam, cycle rides (you can hire bikes in the bungalow park, there is an open air swimming pool open in the summer months. And if you go in the second half of August it is quite a lot cheaper as Dutch schools go back earlier than ours. There are other parks as well run by RCN, but this is the most central.