From what I've read, touring the WWII Sites in Normandy from Paris - you definitely need a tour as the logistics are complex. It's a long way there, and the sites aren't easy to get to using public transportation.
I've been to the WWII Normandy sites twice, both times by bicycle, including last year. I believe that none of the 5 landing beaches are accessible by train. Sword Beach and Juno Beach are essentially beach holiday towns today. They might have bus service from Caen. Gold Beach is somewhat more isolated. The 2 American landing beaches are the most isolated, especially Utah Beach, the westernmost of the 5 landing sites. Pointe du Hoc is near Omaha Beach and is even more isolated.
Both of my visits were during early June, and there are lots of special events around the anniversary of D-Day. I was taken aback by how many town streets, shop windows, and private homes were displaying flags of the Allied countries.
It's a wonderful area to visit if you have your own transportation (a car would be fine), because there are loads of things to see in the middle of nowhere. There are tiny British war cemeteries literally in the middle of farm fields, one had both British and German graves. We frequently came across monuments to specific battles or events on tiny country roads. We saw numerous re-enactment camps. Old US Army Jeeps were all over the place. I happened upon a wreath-laying ceremony at Utah Beach. We saw a new monument being erected at Omaha Beach. I came across a ceremony at Ranville and I spoke with a British veteran of the Normandy liberation, and I met an American Normandy veteran at the Airborne Museum at Sainte-Mere-Eglise. I spoke with a young British soldier in the Ranville cemetery last year who was parachuting in ceremonies at both Ranville and Sainte-Mere-Eglise. He explained things about the British headstones that my friend and I couldn't figure out. He was very funny and told me that he's scared to death to jump out of airplanes, but that the only reason he does it is because they pay him extra!
Both times that I passed through Sainte-Mere-Eglise there were big celebrations taking place. Last year, there was a stage erected and bands playing music of the 1940s. The tourist office was staffed by women wearing 1940s-era dresses and all had 1940s-era hairstyles. The atmosphere was wonderful.
There are some great museums there. My favorites were the Pegasus Bridge Museum and the museum at Utah Beach. The American Cemetery has a good museum, too. The war museum in Bayeux is supposed to be excellent but I ran out of time when I was there. Bayeux is a lovely town, and both the famous Tapestry and the large British War Cemetery (the largest in Normandy) are worth seeing. The American War Cemetery is remarkable, on a perfectly manicured site overlooking the English Channel and Omaha Beach.