A couple weeks before your trip, read up on Key West online or request information from the tourism board of Key West. This will help you and your family get excited about all you will see and do. A lot depends on how old your children are: If they are teenagers, for example, you can rent bikes for them. Small kids, however, will be safer if you just walk with them.
Book a hotel, condominium or other suitable place to stay. "New town" has many large, family-friendly hotels. Some bed and breakfasts in "old town" don't allow children. Find a hotel with plenty of parking; free breakfasts are a helpful bonus when traveling with kids.
Once you get settled into your hotel, take the kids to the pool. Bring your brochures and start planning your adventure. Some kids would be happy to spend the entire vacation in the hotel pool, but there's too much to see and do. Still, no matter how much fun you're having, kids need some down time. If they start getting cranky, head back to the hotel for a couple hours of swimming or just watching cartoons in the room.
Check out the museums. Start with the Mel Fisher Maritime Museum. Especially if you have read about the wreck of the Spanish galleon Atocha, the museum is a wonderful place to spend a couple hours. Kids also will enjoy Ripley's Believe It or Not! museum. Tickets to the Key West Aquarium are good for two days. Make sure and be there when they feed the huge fish in the tank in the back. Don't forget a ride on the Conch Tour Train, a classic attraction for decades.
A good time to wander with the kids is in the morning, after breakfast. Key West is quiet that time of day; you can enjoy the birds singing and the natural beauty of this tropical island. You can play the "chicken-spotting" game -- there are free-range chickens all over Key West. Visit the stores for souvenir toys and trinkets. See the Wyland mural down by the old desalination plant. Get out on the water, too. If the kids are big enough, go out on a snorkeling trip on the reef. If they are small, take a sunset cruise on a pirate ship. They will remember it forever!
Try the local food: Conch fritters won't please everyone, but have plenty of fans. Many restaurants and food vendors offer great fish tacos and Cuban fare. To end the day -- every day -- go to the Sunset Festival at Mallory Pier. In addition to free live music, you'll likely see bagpipers, cats jumping through rings of fire, and of course, a spectacular sunset.
