magnetic funeral flag with “40” on it.
Cover all the windows with sticky notes.
Fake ticket on windshield with a summons to go to court over a delinquent out of state ticket (must coordinate with husband for a realistic state/time frame). Put phone number on it to call to set up court date and give it to someone whose number won’t be recognized but will answer the phone “happy 40th birthday”. If you can work up a script then the person answering the phone can ask for the ticket number and then ask her to verify her license number, address, height, weight, and date of birth (10 years older to the day). The person answering the phone would have to set their cell phone to ring a different ring tone so as to know she is calling and answer as the appropriate police agency.
Crisco shortening on the window. It’s a bitch to get off, and you can write a message in it.
Does her mini-van have a long antenna?
If it does, get a bag of marshmellows, and push them onto the antenna. They do come off, but you have to pull them off one by one, or else cut them off.
Twist open Oreo cookies, and stick the side with the frosting onto the car. You can spell things out with it.
Alright, we went with a saran-wrap sloppy mummy look, lots of balloons tied all over it, and “40!” spelled out in cookies (stuck on w/the cream filling) in the back window.
And she LOVED it! 
Thanks for your help & suggestions! Good thing, to help a couple of moms feel young & mischievous (without causing actual damage).