The Causeway Beach along the road between Sanibel and Fort Myers [1] is actually quite nice despite the traffic zooming by. Parking is free, and there are bathrooms and picnic tables at the main beach area just before the toll gate. The water is a bit rougher than on the island, but it’s shallow, and the sandy beaches are stocked with shells.
Once on the island, beach lovers should head directly for Algiers Beach (Algiers Lane at W. Gulf Rd.). This isolated beach is named after a steamboat that ran aground and was used as a home for a while. A locals’ favorite, there are facilities—picnic tables, restrooms, a paid parking lot—but the beach itself is seldom very crowded. The sand is quite shelly, and the water smooth and shallow; leashed pets are allowed.
Another quiet option is Bowman’s Beach (northwest on Sanibel-Captiva Rd., left on Bowman’s Beach Rd., at the end of the road). The parking lot is a bit of a hike from the actual beach, and the general isolation of the location—it’s quite a ways from any hotels or residences—gives it the feel of an insider’s secret. The beach is fairly popular, however, and the parking lot can fill up quickly, but the white sand spreads out quite a distance from the entry point, so it’s easy to find a quiet spot. There are picnic tables, barbecue grills, restrooms, and an outdoor shower.
Blind Pass Beach and Turner Beach are on either side of the bridge that links Sanibel and Captiva Islands [2]; the former is technically on the Sanibel side, but both are closer to the action in Captiva than in Sanibel. Neither beach is particularly good for swimming as the undertow can be quite strong, especially near the bridge. But that same wave activity also makes for some incredible shelling at both beaches. The soft white sand and crisp blue water are absolutely gorgeous, making it a good place for sunbathing and a near-perfect spot for couples to catch the sunset. Fishing is quite popular at both beaches.
At the northern tip of Captiva Island is Captiva Beach, the only other publicly accessible beach on the island. The wide expanse of soft white sand is quite popular, but the tiny parking lot has the effect of limiting daily attendance. This is one of the most beautiful beaches on either island, so get here early to enjoy the shallow blue waters or to explore for shells. The only facilities are portable toilets.
Links:
[1] http://www.moon.com/destinations/florida/south-gulf-coast/fort-myers
[2] http://www.moon.com/destinations/florida/south-gulf-coast/sanibel-and-captiva-islands