Block Island, Rhode Island
Why go: Block Island, which sits between mainland Rhode Island and the tip of New York's Long Island, is less developed than Martha's Vineyard or Nantucket but has the same New England charm.
What to do: Head to Mansion Beach for good waves and few tourists, and visit the island's enormous cliffs, Mohegan Bluffs.
Where to stay: The National Hotel (doubles starting at $199 in late June, blockislandhotels.com) is famous, but most families rent houses in the interior of the island (try Sullivan Real Estate, blockislandhouses.com).
Bald Head Island, North Carolina
Why go: Cars are not allowed on the island―just golf carts. What's more relaxing than that?
What to do: Visit Eb & Flo's, famous for its pots of steamed seafood. Or stay in and e-mail a grocery list to the Maritime Market―it delivers.
Where to stay: A private house. An average two-bedroom rental runs $2,800 a week in high season, Memorial Day through Labor Day. Renting through Bald Head Island Limited (baldheadisland.com) gets you free access to the Bald Head Island Club, which caters to golfers, and to the beachside Shoals Club.
Catalina Island, California
Why go: Catalina has some of the best snorkeling in the United States, and its famous colored tiles (created by local artists in the 1920s and 1930s) decorate everything from fountains to trash cans.
What to do: Shop and eat in Avalon, the island's only city. Hike the many trails or ride horseback. And stop by the airport, a bona fide tourist destination, high on a mesa in the middle of the island.
Where to stay: The Hotel Villa Portofino has a sundeck with views of Catalina Harbor (from $116 May to October, hotelvillaportofino.com).
Isle of Palms, South Carolina
Why go: A 20-minute drive from Charleston, it's a cinch to get to. City amenities (grocery store, high-end restaurants) are not far away.
What to do: Play golf or tennis, and sun yourself on the beach at Wild Dunes, the largest resort on the island. On Sullivan's Island, 10 minutes away by car, you can watch the sunset while dining on the patio of the restaurant Atlanticville.
Where to stay: The Wild Dunes resort offers private villas in addition to rooms at its Boardwalk Inn hotel (two-bedroom villas start at $430; summer rates for doubles at the Boardwalk Inn start at $195, wilddunes.com).
Little St. Simons Island, Georgia
Why go: A hop away from the more popular St. Simons and Sea Island and home to only one lodge, it's about as close as you'll get to having your own private island―no phones, no e-mail, few tourists―within the United States.
What to do: Enjoy your solitude on the beach, kayak, or consult the naturalists who work on the island. They give guided tours to the loggerhead-turtle nests that dot the beach in summer.
Where to stay: The Lodge on Little St. Simons accommodates just 30 people at a time. The double-occupancy room rate (starting at $625, littlestsimonsisland.com) includes all meals and activities.
Mackinac Island, Michigan
Why go: It's one of a few islands not on a seacoast; it's in Lake Huron. Another plus: no cars.
What to do: Take a tour in a horse-drawn buggy or "rent a tandem bicycle―it takes you back to your childhood," says Stu Stuart, a comedian who lives and performs in Mackinac (pronounced Mack-i-naw) every summer. Stop at the Cannonball Drive In, a hot-dog stand known for its fried pickles. And don't miss the Grand Hotel's front porch, the longest in the world.
Where to stay: For families, the Lilac Tree Hotel has suites with two TVs apiece (from $200 in the summer, lilactree.com). "But Bay View bed-and-breakfast has the best cinnamon rolls," says Stuart (from $97, mackinacbayview.com).
Why go: Marathon is a quieter alternative to Key West and only a two-hour drive from Miami.
What to do: Drive north and south on the road that links the Keys, gazing at less developed islands. Stop for breakfast at Leigh Ann's coffee shop on your way to the Dolphin Research Center, on Grassy Key, 10 minutes from Marathon.
Where to stay: Tranquility Bay has two- and three-bedroom beach houses (from $1,794 a week, tranquilitybay.com).
Orcas Island, Washington
Why go: Orcas, the largest of the San Juan Islands, has two big sights: Mount Constitution, the highest point on the islands, and whales. "During the summer, you see whales every day. It's a thrill," says Jeremy Trumble, co-owner of the Inn on Orcas Island. Bonus: The San Juan Islands don't see much of the rain that hits the mainland and get half as much precipitation as Seattle.
What to do: Visit the art gallery at Crow Valley Pottery, in the village of Eastsound. And sample organic food at Christina's Food & Wine restaurant. "If Christina can get her ingredients from the island, she does," says Trumble.
Where to stay: The Inn on Orcas Island (doubles from $145, theinnonorcasisland.com).
Sanibel Island, Florida
Why go: The beaches of the Gulf Coast have some of the most beautiful white sand in the world.
What to do: Get up early and look for shells on the beach. Because of the beach's east-west orientation, shells wash up by the bucketful. Keep a lookout for junonias, rare conical shells with brown spots. "They are very prized. When someone finds one, they get their picture in the paper," says David Meardon, a photographer who has lived on Sanibel for 25 years. Hungry? "The Lazy Flamingo has the best mesquite grilled–grouper sandwich you can find," says Meardon.
Where to stay: The Sundial Beach & Golf Resort (doubles from $152, sundialresort.com) offers kids' activities and games.