We just visited Sanibel for the first time and we chose to stay at Mitchells Sandcastles. I based our choice on the reviews on Tripadvisor and price. We stayed for a week - 6 nights in Sea Chest and then 1 night in cottage #4 (for the same price) because Sea Chest was only available those first 6 nights. Sea Chest was small, but nice. We would definitely stay here again, but would 'upgrade' to something a little bigger like #4. Just enough extra space, windows, etc that we liked it better. Everything was very clean, fresh towels on demand, anything we needed, we got from the front office. Good restaurant recommendations, too. Beach chairs, umbrellas, bikes all available to guests at no charge.
Our only complaint was to Mother Nature and the red tide - that made the beach a little gross.
Mitchell's Sandcastles is away from the hustle bustle, but very close to everything you need to get to. It's quiet, mostly older retired folks but a few families with small children. The sunsets and shelling were amazing and changed every day. We brought home way too many shells!