Scare up at least one cardboard refrigerator box -- preferably two or three, so the neighbors' kids can join the club. These giant corrugated towers are sturdy enough to hold up over time and they're wonderfully tall so kids can stand up inside, unlike the ubiquitous tent with its close quarters and low ceiling. Line up two or three boxes with open flaps down and fasten them together with plenty of duct tape.
Turn the multi-box clubhouse structure upside down and use a box cutter, saw or large craft knife to remove the double cardboard walls between the linked boxes. Save the panels in case you plan to add a door. Seal the interior clubhouse with more duct tape. Now the shell of the clubhouse is complete and ready to be painted.
Measure, mark and cut out one door and several windows using the box cutter. Wrap strips of duct tape around the openings so clothing doesn't snag when kids enter the clubhouse or stick their hands out the window. Here's a tip to make sure the window and door heights match your child's size: Ask your tallest child to stand in front of the clubhouse. Use his body as a guide for measuring and cutting out the door frame. Do the same for the windows.
Allow the kids to pick paint colors for their clubhouse. If they've given the structure a name -- like "Fort James," "Tiffany's Palace" or "Paulie's Pirate Cave," there's a good chance the theme will set the tone for the colors. Give the entire clubhouse one coating of quick-drying paint so the cardboard doesn't absorb too much water. Add a second coat and once it dries, get busy decorating the exterior or turn the job over to the kids if you're fearless. Don't forget the clubhouse name. If you don't want to letter it on yourself, use large stencils to do the job. Apply a sealant to finish the exterior.
Sew curtains and a curtain door using old sheets or sturdy fabric. Iron-on tape makes quick work of hems and you can attach the curtains to the insides of the clubhouse windows with Velcro so you're able to remove them for washing as needed. If the kids want a "real door," cut one from the cardboard you removed when you joined the boxes together. Mount it with hinges. Don't forget the all-important warning sign on the door: "Members Only -- Everyone Else, Keep Out!"