On a Friday morning in summer, cars on both sides of the main road—actually, the only road—connecting Sanibel with its smaller sister island, Captiva, came to a halt. A turtle had stopped in the middle of the thoroughfare. No one honked. Finally, a man got out of his truck, picked up the turtle and carried it gently across the road.
 
That laid-back spirit—as well as slashed hotel rates—is why summer might actually be the best time to visit Sanibel and Captiva, two barrier islands off the southwestern Florida coast near Fort Myers. “We call it the ‘second season,’ ” says Barb Renneke, co-owner of the Captiva Kayak Company and an islander for 23 years.

The summer downshift seems to affect everything on the islands, home to some 6,300 full-time residents. The ocean changes from a drab winter gray to the sort of brilliant turquoise that lures so many to the Caribbean. And all that water comes in handy when summer temperatures hover around the 90-degree mark.

Things might slow down, but there’s still plenty to do, starting with the most renowned pastime on the islands: shelling. To track down the more coveted specimens, consider hiring a guide like Mike Fuery to take you to remote shelling beaches by charter boat (239-466-3649; $225 for three hours). After a half-hour boat ride you’ll arrive at a serene shoreline littered with shells left exposed by the tides. (The shallow water here also makes for easy snorkeling.)

A kayaking trip on nearby Pine Island Sound is another good outing. Though the sound is 20 miles long and 4 miles across, it’s not deep: you can see clearly to the bottom in most places. In a kayak you’ll glide over crops of sea grass that look like a tropical meadow.  Dolphins pop up regularly then sink beneath the water, while manatees occasionally surface with a small roar of bubbles (Captiva Kayak Co.; 877-395-2926; captivakayaks.com).

Thunderstorms roll in most afternoons and out by early evening, which means mornings are best for outdoor activities. If you decide to go kayaking, “show up at nine a.m. and be back in by one p.m.,” Renneke advises.

Rain or not, there are plenty of post-lunch options. Periwinkle Way, the main road in “downtown” Sanibel, may be a parking lot in high season, but it’s mercifully free of congestion in the summer. That’s because visitors and residents alike are usually out riding the island’s 23 miles of scenic bike paths (Billy’s Rentals; 239-472-3620; billysrentals.com; all day, $15). If rain turns heavy, duck into the Sanibel Bean, where everyone meets for coffee or sandwiches on the old-fashioned porch. With its cheerful marigold-colored walls and Wi-Fi, the Bean can easily become a daily hangout.

Conservation is critical to these islands, which is clear when you visit the J.N. “Ding” Darling National Wildlife Refuge. It costs $5 per car to take the four-mile Wildlife Drive, but it’s worth it. One designated stop along the route is the Shell Mound Trail, a boardwalk that slices through a thick hammock of tropical trees, with lizards skittering everywhere. Another natural attraction, the Sanibel-Captiva Conservation Foundation, offers guided birdwatching outings, nature trails and a butterfly house. (The Foundation’s programs aren’t always operating in summer, so check before you go: 239-472-2329 or sccf.org.)

At the Sanibel Historical Village & Museum, exhibits explain island history from its formation more than 3,000 years ago to construction of the causeway in 1963. The Bailey-Matthews Shell Museum (239-395-2233; shellmuseum.org) is the place to go before that big shelling tour: Every shell type you’re likely to find is on display, along with engaging exhibits like one on deadly sea creatures (the golf-ball-size blue-ringed octopus, for example, has toxin 10,000 times more potent than cyanide) and another on Sailor’s Valentines, gorgeous shell art made by Barbados women and bought by 19th-century English sailors to take home to their wives or sweethearts.

For a shopping break, try Sanibel’s Periwinkle Place, with its boutiques, spa and covered walkways. Traders restaurant is also a store with an eclectic mix of imports. At Sanibel Goldsmith Gallery, you can watch custom jewelry being made. Prices top out at $12,000, but smaller, less jeweled items can be had for as little as $40 (800-472-7572; sanibelgoldsmith.com). On Captiva, Jungle Drums Gallery offers whimsical wildlife art.

The lack of crowds is a summer perk, but the real gift is the deepened sense of community that even visitors will experience. “There’s much more of a hometown feel in summer,” says Melissa Rice, a real estate investor. “Store owners can take the time to talk to you.”

Jim Sprankle, president of the “Ding” Darling Wildlife Society, agrees. “There are sand beaches all over the world,” he says, “but you’ll never find people like you’ll find on these two islands.” What kind of people? Well, people who’ll smile at an indecisive turtle stopping traffic on a breezy summer morning.