This activity is very easy- and you only need things you already have in the house! Grab a few blankets and a couple chairs and you can make an awesome fort for the kids! Admittedly this doesn't happen in our house as often as they would like. I always end up with 2-3 loads of blankets to wash after they get done playing in it, so I limit the fort making to once every couple months.
We make one big fort either in the living room or under the dining room table, and then I let it stay up for about a week. They have gotten very creative, using clothespins and chip clips to hold the blankets together so they don't come apart. They love for me to get in it with them. We play board games and watch movies in it. Sometimes it's the simplest things that make them happy!