What's in the bedrooms that will belong to future babies?

We currently live in a 2 bedroom apartment. We have an offer in on a 4 bedroom + den house and currently only have one kid. So if we plan a guest room and office in the den, we still have one completely empty bedroom. I obviously don't want to spend a fortune furnishing a room that will (hopefully) only be empty for 1-2 years. So I'm curious what others have done with extra rooms when upgrading to a house with room to expand. Just leave it glaringly empty??