I would recommend an add on trip of 2 or 3 nights to explore Normandy...we did that about 10 years, and will be returning for the same idea next June.
In either case, I would suggest looking into Overlord tours, mentioned here frequently, whether it be from your river boat, or while on a land trip. They are awesome, offer a number of tours. We used them for an 8 hour tour 10 years ago, and plan on using them for an 8 hour and a 6 hour tour next June.