What's the best way to get from Disneyland Europe to the Normandy invasion sites, either Caen or Rouen?
Take the RER A from Marne-la-Vallee - Chessy (the Disneyland station) to Gare de Lyon in central Paris. Transfer to Metro line 14 and take it to Gare Saint-Lazare. Take a SNCF train from Gare Saint-Lazsre to either Caen or Bayeux. Some trains go directly to Bayeux. Some require a connection in Caen. Rouen is nowhere near the Normandy invasion sites. However, trains from Paris to Rouen depart from Gare Saint-Lazare.
Rod, Bayeux is perhaps the best "home base" for visiting the D-Day beaches. When you arrive in that area, you'll either have to rent a car or use one of the excellent local tours. I would highly recommend the tours. They take care of all the transportation, and you'll learn far more than if you just wander around on your own. If you need information on some of the tour companies, post another note. Happy travels!
You can also stay in Caen for the Caen-Normandy Memorial Centre for History and Peace, a newish museum which also runs a guided tour over to the Normandy beaches. http://www.memorial-caen.fr/portailgb/index.php?option=com_content&view=article&id=798&Itemid=1528