Hi there,
It's actually hard to give a precise answer because the price range is quite wide, and it also depends on what kind of meals you enjoy (starter, main course, dessert twice a day, or one good meal and not much else for the rest of the day...).
It also depends on where you'll be staying.
But generally, most hotels accept credit cards, so if that's the case, you can pay for your activities, rooms, and meals at the hotel with your card. If you have cash in euros, hotels can exchange it—some offer a fair rate, while others are a bit less favorable. Some restaurants might accept euros or credit card payments, but again, it depends on where you go. The cheapest spots and smallest eateries will only take Ariary.
For generous meal budgets, if you have two decent meals a day for two people, I’d estimate around 60 € per day (2 x 2 x 15 €), so roughly 400–450 € for a week. Of course, you could spend more or much less—this is just a rough average.
Where will you be staying on Île aux Nattes?
As for exchanging money, I’d recommend doing it at the airport when you arrive in Tana. There are also ATMs in Sainte-Marie (but not on Île aux Nattes—you’d need to go to the small town where the pirate cemetery is, which involves a bit of logistics: 1 pirogue ride and then a tuk-tuk). You can withdraw cash with a credit card there (I can’t speak for all banks, but for us, the withdrawal rate wasn’t too bad).
Happy planning!