While it's not adults only, I would recommend Camping Le Cormoran in Ravenoville Plage, nr. St Mere Eglise (the first town in Normandy to be liberated). It's at the northern end of Utah Beach, and the beach is right outside the gate. It's in easy reach of all the museums and attractions at that end, and access is very easy. We've been twice, both times in August, and, although full, the place never felt at all crowded or busy, and we didn't even notice the children on site. Facilities were ample, and spotless. A fascinating area, a great site, and well worth a visit.
The site is Caravan Club listed, and I've reviewed it on www.caravansitefinder.co.uk . It has its own website at www.lecormoran.com .
Whatever you decide, and wherever you stay, have a great time. The locals have never forgotten how the Allies liberated them in the 1944, and they make Brits and Yanks alike very welcome.
Museums are all good. The best we found were at Arromanches. The glider museum at St. Mere Eglise is also very good, and there are many others.
Advice... If you are visiting more than a couple of museums buy a 'Normandy Pass', the discounts you get are well worth it. Reception at the site will advise you, or you can buy them at any attraction.
Enjoy!
(By the way, I'm not the same Derek as he who posted further up the page, although he too has a brilliant name!)