We spent a week there in early March and it was fun! No beach swimming but we had a couple days at each end of our trip that were warm enough midday for the heated pool & hot tub. Besides that, we enjoyed the naturalist-led swamp walk, learned to throw a shrimp net, played goofy bingo, saw a very good magician, painted fish print t-shirts, played shuffleboard, billiards & indoor board games, walked the beach for seashells, rented sea kayaks (close encounters with dolphins in broad creek), took a gourmet cooking class and made a few new friends over dinner together in our room. If weather permits, try the evening campfire sing-along & s'mores.
Your children might also enjoy the scavenger hunt, looking for signs of "Shadow" (their mascot golden retriever), evening bedtime stories with cookies in the den and other activities geared toward the younger set as well. There are hammocks and a playground at the resort too.
Fishing and crabbing are futile in winter there, with the season resuming in mid-April. There are also dolphin watch cruises, horseback riding, bicycle rentals, nearby golf/tennis and other "for fee" activities. The dens (at the resort and at the beach house) both have fireplaces so don't hesitate to wander the resort, even in bad weather. While there, call ahead before walking to meet for an outdoor activity. When the weather turns bad, they replace outdoor activities with indoor group games.
Our week was an exchange via II (timeshare). We enjoyed our full week, though we just love HHI and have been to various resorts on HHI. 4-5 days in wintertime would be a terrific vacation if you are not sure whether you will find enough to do on HHI with your family in the winter. Keep your expectations open... ask them to fax you a recreational schedule ahead of time... remember that it's an upscale, yacht club-country club atmosphere on the island, with a relaxed, family-friendly theme at the Disney resort. Have fun!
