Fun ideas for rainy days?

okay, me and my cousin usually don't have a difficult time coming up with crazy and wacky ideas to do on rainy days, or any days for that matter; we've put a slinky down and up escalator, we've filled water balloons with paint and threw them at each other (uber amounts of fun by the way), we've had a nerf fight in wal-mart, ect. but we cant seem to come up with anything good for the past week. got any good ideas?