Kids love playhouses and they are great to have indoors.  You can make one yourself from cardboard, of course. But sometimes it's worth it just to set up a pre-made playhouse and start them playing.  I love that these come ready for kids to draw on and decorate.  Even if you reinforce with duct tape (often a good idea, especially for a birthday party or rambunctious three year olds), you can cut the duct tape when you're done, fold it up, and store it.  Pull it out again in a couple of months for another round of imaginative play.