Luggage and bags:
There is no luggage storage anywhere in town - except for your hotel maybe, so if you can avoid bringing a bag on wheels, take a rugsack or other sort of bag you can carry. This is because it's a pain to drag a trolley bag along the cobbled streets and this may not be good for items such as lap tops - especially if you arrive in Saint-Malo in the morning and have to spend quite a few hours in town before being able to check in to your hotel in the afternoon.
Clothing/Shoes/Weather Gear: If you go in Spring, bear in mind that it can be very windy at the seaside, so take something warm as it is not very pleasant to feel cold if you were hoping to spend some time outdoors (walking along the walls surrounding the old city or visiting the tidal islands). The streets in the old city are cobbled, so bring some sensible shoes too.
You really only need a bathing suit and just the bottom is enough for the girls as St. Malo is a topless beach.
Photo Equipment: Lots of film for the views