If you go to Normandy to walk the beaches in December you won't have to worry about crowds. Of course, many of the museums and sites will be closed for the winter. But you can still see many of the monuments, and the AMBC American Graveyard will be open. The weather is going to suck.
You can, with some luck and perseverance, visit most of Juno, Sword, and Gold in a day. But the monuments are right up in the towns, and take a bit of hunting down. If you do go this way make sure you stop in the Canadian Cemetery.
Utah and Omaha are further out, and quite different. Utah is in the middle of wide open dunes, Omaha has the large seawall and cliffs. Again, you can visit both in a day, depending on what else you add other than just walking the beaches.
Caen is good for the British/Canadian sites, but a fair drive from the US positions.