I have only been to the Paris area once so far (I'm currently planning my second visit this summer!), here's what I can say from my limited personal experience:
Monet's home & gardens in Giverny is a MUST! Definitely DO go to Giverny on the way to Normandy. You will feel like you stepped into one of his paintings while touring his water garden! I'd plan to spend at least 3 hours for that wonderful experience. We took my 7 and 11 year old here in 2009. My daughter eventually used the experience for a book report on Giverny, complete with pictures of herself in front of Monet's home and in his gardens.
We also went to Mont St. Michel, arriving early evening and staying overnight on the tiny island. It's true there is not a lot to do/see on MSM, but what is there is - is magnificent (in my opinion). Just make sure to get off that main drag with all the little trinket shops! The experience is especially wonderful if you can wake up early in the morning (as I did) and have the place to yourself. I think I got up around 6 am and got to walk around all by myself for over an hour and watch the sunrise over the mudflats. My husband and I then got in line to get into the cathedral before it opened. The cathedral is a MUST as well! Arriving early evening when we did was also exciting; there were seagulls flying over the rooftops as dusk settled in, giving the place a most eerie and mystical look. I was entranced by Mont St. Michel, so much so, I'm going to visit again next summer since I'll be spending two weeks in Brittany in a home exchange.
The little bit I saw of Normandy in 2009, I fell in love with! We stayed in a charming B&B called Le Mas Normand (it was in R. Steve's guidebook). It was the first time my kids stayed in a B&B, and they absolutely loved it. My daughter especially loved the charming room with a view on the yard with cows, the delicious breakfast in the pretty sun room, and the owners dog who wandered the grounds. We loved the little town of Arromanches with the 360 degree WWII movie, the remains of Winston Artificial Harbor on the beach, and just the charming town center. The American Cemetary and it's museum was very moving and beautiful. We visited several of the landing beaches. It was a great history lesson for my kids; but it was also a fabulously beautiful area. The latter really surprised and delighted me.
I have not been to Dinan yet, but am looking forward to visiting it myself next summer.
I'm not great at critiquing peoples schedules because we like to take in the sights very, very slowly.