I think that kids like forts and play houses so much because it's their own private place that grown-ups can't enter. Or they can, but it's usually a tight squeeze and I always leave with a new bump or bruise. Here's a clever gift idea from a regular, green-thinking dad who decided to build his daughter a house out of cardboard from old appliance boxes.
See it up top? How cute is that! It took him about a week to construct it using pieces of cardboard that he hid different places around his house and simple tools that most people have. He surprised his daughter with it on Christmas morning. You could always get them involved and make it a project for both of you after the holidays.
My kids are 5 and 6 and they STILL love playing games in little houses. They really want a tree house -- that's our next endeavor for summer.
You could probably build something like this yourself, or if you are totally at a loss, order the plans for the 48 by 36 inch house for $20. The plans include patterns that you trace on the cardboard, cut out with a utility knife, and fasten together. You paint it yourself using your own colors.
Would you consider making a playhouse for your kids for the holidays?