I'm a fan of Great Wolf Lodge, having visited one of their locations back in January (and had an awesome time!). Great Wolf Lodge is having a special 48 hour sale starting today (Tuesday) at 10am. The sale is good on stays from September through December (the Snowland Christmas decorations are gorgeous).
Great Wolf Lodge is offering the lowest rates all year with this sale. Check the website to find when your location's 48 hour sale starts (today's is for Traverse City, MI and Pocono Mountains, PA, tomorrow's is for Williamsburg, VA and Kansas City, KS).
So if you're looking for a fun getaway during those not-so-warm months, Great Wolf Lodge is a wonderful destination. Their indoor water park is especially appealing for little ones. They have locations across the US, so find the one nearest to you!