Whip cream (even reddiwhip) will go flat, it won't stay light and fluffy for long. It might also damage the handle or paint.
Use aerosol shaving cream (not gel), it's just soap.
I'm a big fan of April Fool pranks, here's a great link to car fun on April 1st. http://www.discoverfun.com/pranks/cars.html
just don't use shaving cream . it will screw up the paint. if you WANT to screw up the paint, stick bologna all over the paint in the hot sun. when it's pealed off, it'll take the paint with it! but if you don't, stick to window chalk. and also, wrapping plastic wrap around the car is good. if looking to go under door handle, put vaseline under the door handle so as to make it hard to grasp and open. also if you cut a potatoe in half, and stick half in the tail pipe (all tail pipes) it will make it very hard to start the car, and it will not stay running untill the potatoe blows out. and when the potatoe blows out, it will make a very loud sound and scare them. lol. best of luck with your pranking.