I really enjoyed Saint-Malo when I was there in April on a Road Scholar tour. We were there 3 nights in a hotel on the beach just outside the city walls. We spent a full day in Saint-Malo and I did not have enough time to see everything I wanted to see. The group had an excellent walking tour of the old city but in my free time I only got to 2 out of 3 islets (tide was still in over the causeway on the 3rd one but got to Grand Be and saw the unmarked tomb of the writer Chateaubriand). I did see the cathedral where Jacques Cartier is buried. However, I also did not get to the 39-45 Bunker Memorial (just outside the city walls), or tour the Corsair's house (in French but the ones that went felt they were able to figure most things out), do the complete walk around the walls (got about half-way), or the Museum of Saint-Malo in City Hall or take the shuttle boat over to Dinard (some Alfred Hitchcock references in this village if you are a fan).
I was mesmerized by the tide changes here. It is such a huge swing and wow, it comes in fast. I walked on the beach once but sort of freaked out when I realized how fast it was coming in and that I wasn't exactly sure where the next stairs up were. I was fascinated by the public swimming pool they have on the beach that is covered by the tide. When the tide is fully in you can only see the diving board sticking out. According to the walking tour guide, it was built to teach those who were going to be sailors how to swim.
We just spent a few hours at Dinan. There were several buses full of high school age students visiting as well and the old city area felt very cramped.
I would actually stay in St-Malo as well, just to see how the tide affects the beaches and islets over time. So very interesting-at least to me!