Start your day by getting to the market. The centre is very small you can do everything by foot. If you want it is always possible to rent bikes. The yellow bikes are available in the centre to go around the city, now you can even rent an electric one.
This is a very old market, you can go inside and find anything you want for your shopping groceries. On the right you have the fish's area, in the middle you'll find cheese and meat and on the left vegetables. On Saturdays, all the best vegetables are outside and they are all from the region and handmade produced. I will recommend buying oysters there (ask for the Roumegous green ones, they are my favourites) and a little glass of vin blanc. You can enjoy your wine around the market or a cup of coffee of course.
The city is more than 1000 years old so enjoy a walk in the streets, under the arches and the pavement. Don't miss the City Hall, the market streets, the St Nicolas Street and the Grosse Horloge. You will very quickly find yourself on the Harbor. This is the heart of the city, you can have a coffee, enjoy the sun and the view on the boats. At nigh, it is a very populated area, only by foot with music, art shows, concerts, mainly during summer season.
Then you have several solutions to get to the Ile de Re. You can take the bus, you can rent a car or do it as locals, you can rent a bike.
You have to pass the bridge which is 3km long and then you are on holidays. The smell of the pines, the dunes, the beaches, everything is very comforting. It is a 10 km ride to Saint Martin de Re through forest, along the coast and between the vignes. When you get there, you won't miss it because it is a fortress build by Vauban. Find your way to the old village inside the walls then you will arrive in the harbour. It is a very cute one with nice boats and nice shops around. You will find a lot of merchants selling waffles and candies but the one you should target is " La Martiniere" ice cream shop. The best in the area. Don't be shy, try them all. My favourite are Pecans, Almond milk and Pralines, Pear and the one and only Carasel which is salted caramel.
If you are more about sea shells lets drive yourself to "le Tout du cru" restaurant, with nice sea food and oysters to share with goats cheese and white wine.
When you leave Saint Martin, you have a lot of options for the day. One of it is La Couarde en Re, a nice and very cute village.
On the way to get there, if you are cycling, you can stop at one of the many oyster's locations. They are oysters producers, and you can stop there in those little cabins, to taste oysters straight from the sea with wine from the Ile de re and goat cheese on bread. This is the way you enjoy oysters here. Of course, they also have other seafood and conserve if you want it as a souvenir.
La Couarde is known for it's really cute village and also its proximity to the Marais Salants, which are the place where they hand produce salt. You can visit the Marais which are composed of different basins of water where they swipe the water every day to collect only the crystal at the surface. There you can buy salt, and Fleur de Sel, which is the very fine salt on the surface. You will also find caramel with salt inside, taste it, it is delicious.
Finally to really enjoy the area, you can go to the old market of the village around the church and lay on the beach of Les Aneries, you will find a nice stop to bathe before heading back to La Rochelle !