Steve,
Re Rouen, yes, you could do as a day trip from Paris, but take the earliest train there and the latest train for return, to give yourself a full day there. Visit the cathedral that so fascinated Monet, walk around the medieval quarter (beautifully impressive), follow up on some of the Joan of Arc history/sites, eat well (Julia Child's first meal in France was here and became her inspiration to become a chef). There are several museums....Beaux Arts, antiquities, iron arts, ceramics to name a few. If you choose to drive at all, the road (D982 I believe) between Rouen and Bayeux is a wonderful drive next to the Seine River in the countryside and you can take the Route Des Abbayes. Visit the spectacular ruins of the Abbaye Des Jumieges.
Re Mont St Michel, taking Rouen out of the equation for Bayeux, your 3 days/2 nights in Normandy is possible, but a stretch. Say, an early arrival in Bayeux will give you most of a day and you could do a half day tour of the beaches or see the town (museum, cathedral, the town itself). Next day an MSM tour from Bayeux which will be an all day excursion. You will be there when it is more crowded, but I have been there mid-day and I still appreciated the visit. Just pack your patience in your day pack. After your second night you could do an all day beaches, etc. tour before an evening return to Paris.
pbscd makes some really good suggestions, as do many others here. If you decide on a day trip to Rouen, you don't need to pay for a tour, Get your train tickets yourself (double check your schedules). The medieval quarter isn't huge and is eminently walkable. We did it at the ages of 73 and 74, and that was before I had either of my much-needed knee replacements. Just a thought.
Amusez-vous bien!