Invite her over to your house for some late night lovin. Also invite a guy friend over but don't tell her. Have the guy hide in the closet. lay her face down near the end of the bed and make sweet love to her from behind while standing up, (very important to be standing).
The Suprise:
When things start to get hot, signal for your friend to come out. Have him slip into position and take over. Then say to her "How's that feel, is it good, etc..." When she replies Jump in front of her and yell:
"SUPRISE"
