I very much enjoyed the one-day van tour of invasion sites I took in 2017, but it was a full-day tour that didn't leave time on the same day for sightseeing in Bayeux other than walking around the historic center. The town of Bayeux has a lot of other offerings to enjoy: an excellent invasion museum (you could spend hours there), the cathedral, the tapestry, a walking tour offered by the toutist office, and I believe there's a small lace museum or something like that. So a return to Bayeux for a day of sightseeing is very worthwhile.
Since I stsyed in Caen rather than Deauville, I'm not sure about the travel times you'll face. I just don't know whether Etretat is practical as a day-trip without a car, but you can definitely get to Honfleur (bus), Cabourg (also bus, I think) and of course Trouville (city bus or even walking).
So I don't think you'll have any trouble filling your time. If you do, nearby Caen is a rather large, industrisl city basically flattened during the war and thus rather short on historic charm. However, it has two old abbeys and the very large Peace Museum, which can be a full-day affair if you're interested in the causes of WWII and the Cold War aftermath as well as the war itself.
A short train ride from Deauville is the major pilgrimage town of Lisieux. I haven't been there, but it seems to have some historic architecture as well as the important cathedral.