Do you recommend any tour companies for day trip to and fro central paris to the ww2 sites in Normandy?
I have one scheduled in a few weeks with Bayeux Shuttle. They have info on which trains to take and even have a link for you to buy the train tickets from Gare St. Lazare. I booked this trip based on recommendations on fellow RS travelers suggestions. The maximum number of people on the tour is 15.
I know there are companies offering one-day Normandy tours from Paris. Probably some of them go to Omaha Beach. The problem is that Omaha Beach is 170 miles from Paris, one way. You would be sitting on a bus for more than 6 hours. If you're seriously interested in the D-Day sights, a day-trip from Paris is not the way to see them. If you're not seriously interested, a tour from Paris isn't a good use of your time, and I'd recommend skipping Normandy on this trip and seeing more of Paris (or some other destination you have planned).
Assuming that Normandy is important to you, I highly recommend taking at least one night away from Paris. Go up to Bayeux by train the night before you take the tour. On weekdays there's a 6:09 PM train that gets to Bayeux at 8:19 PM. Figure out what you'll do with your luggage: Either take just a small tote bag to Bayeux (and check with the tour company to be sure you can leave it in the van during the tour) or arrange with your Bayeux hotel to leave your luggage there until after the tour. The second approach will make things tighter for catching a train back to Paris at the end of the day, because you'll have to swing by your hotel to pick up your bag. Return trains are at 6:35 PM (probably too early) and 8:09 PM (arriving in Paris at 10:26 PM.
Of course, you could stay two nights (or longer) in Bayeux to see the WW II museum, the tapestry, etc. There are lots of other things to be seen elsewhere in Normandy as well. I got around pretty well even without a car.
I used Overlord Tours, which is often recommended here. Others have mentioned Dale Booth. There are additional options. My full-day tour cost about 100 euros last summer. Most if not all of the Bayeux-based tours use small vans, not big tour buses.
Train fare between Paris and Bayeux would be between 30 and 89 euros round-trip, depending basically on how early you buy your ticket. You can check schedules and fares on the SNCF website.