A harmless prank is the best way to create a laugh. Have you tried something too silly to scare her? Plan an act to scare your girlfriend and it's worth it to see her reactions.
They say it's very easy to scare a girl. Just for fun, here are some suggestions.
Guys, go for it.
P.S: Don't forget to hug her tight after that!
And don't forget to kiss on her forehead after this mischief.
This will truly work.
Make some scary noises as well.
Scare her telling that you saw her with her ex.
Make sure you act naturally.
Let it be in front of her when you both sit.
Don't hurry, take your time and make it really very scary!
Don't give her a clue that you are acting.
A dark room is the deadliest place to scare her.
Behave abnormal and show some fierce madness. Be a good actor, then.