Great ways to scare someone in their home?

HELP! So my husband and I want to scare my younger cousin in his home. We dont want to break in or anything but he got us with a good prank and we want to scare him at his home. He has 3 roommates but lives in a house and his bedroom window is in the back ... any good ideas? He's 21 so it'd have to be good .. no spooky halloween sounds or anything .. HELP!