It needs to be difficult enough to remove that she can’t get it off before she goes to work, but not permanent or damaging.
My dear friend is turning 30 and I’ve decided to expand on the “Over the Hill” theme by making her 40. This will baffle her many co-workers and necessitate much explaining :D.
Is it hard to get shaving creme off a windshield?
I was thinking I could tie lots of things onto her mini-van (like big black cardboard "40"s) & use metal twisties, so she can’t just cut them off.
You could probably wet it down then cover it in confetti.
Oooooh, I like that! That would take FOREVER to remove!!!
I know you’re not asking about the advisability of your plan, but do you really want to do this to your friend? If it was me, I wouldn’t be amused, I’d be really annoyed I had to field a bunch of questions and spend time cleaning my car to undo your little prank. It seems a bit mean spirited to me.
Naw, she has a great sense of humor. I ran this past some mutual friends (and her mother) and they agreed she’d get a kick out of it.
Though I definitely DON’T want to risk any permanent damage to her vehicle, that would NOT be cool.
Part of why this’ll work is I’m a lot older than she is, I recently turned 45. So 30 is still really young, to me. Hopefully it will be to her, too, once she gets 10 years back LOL!
I’d be worried about the confetti idea… I had something similar done to my car for my first wedding and once the confetti dries, it can be difficult to remove w/o damaging the paint.
Wrap the entire van up - either in foil, brown paper or wrapping paper - hey you could always use a giant bow too like she’s being given a car at 16. If you can get her keys for a bit as well, you can wrap everything in the car as well individually.
We did this to a very anal person at work (with a sense of humour) - wrapped his entire desk in brown paper. Took him forever to unwrap things but there was actually no mess.
Can you write in silly string?
dont know if this damages paint or not.
I’d be afraid to use shaving cream. If the interwebs are to be believed, it can damage the paint. They do make special paint markers that can be used on car windows.
I was at a wedding once where the groomsmen filled the car with balloons and then wrapped plastic wrap (Saran Wrap type stuff) around the car. I don’t think any damage was done, but the plastic wrap might take a few minutes to get off. They used quite a bit of the stuff.
Ooooh! I’m liking that wrapping paper, too!
Hey, I could put the confetti IN the wrapping paper, then it won’t stick to the paint!
Her husband’s in on this with me, and her 3 kids are going to be tickled. tee-hee! 
I have a couple that depend on the weather. If it won’t rain for a few days, get some scented dusting powder, the cheaper the better. Pour it into the cowl vent where the heater air is sucked in. Wipe off the excess, so it won’t show. When the car and heater in turned on, a big sweet, smelling cloud will engulf the inside of the vehicle.
If it will be dry for a few days, and your friend isn’t tall enough to routinely see the top of the minivan, spray a square of adhesive to the roof. While it’s wet, cover the square with grass seed. When it rains, the seeds will sprout. In a few days, there will be a little putting green up there. If you don’t want to do it on the finish, do it inside the wheel wells. There will be a little lawn around each wheel.
I saw a group of guys putting baby oil and glitter all over a car once. Sounds like a pain to remove, though. I vote for saran wrapping her car.
What happened to white shoe polish in the windows?
Once upon a time, the preferred ink for writing “Just Married” etc on cars was white liquid shoe polish - the kind with the foam applicator built into the bottle. Works just like a pen, washes off just fine. I have never heard of damage to the finish.
ETA: Damn!
You might need some expert help on this, but how about rigging something up so that when she reaches 40 miles per hour the first time, if she drops below 40 miles per hour, her car will explode in a huge fireball? I saw someone do this in a movie once and it looked like a lot of fun. Of course you will want some hidden video screen system in the car that pops up and gives her a warning once she reaches 40 mph (your face would appear on the screen along with her family and the other people in on the prank, and you would tell her about the bomb and then sing Happy Birthday).
ETA: once she stops the car, you would hear a big bang sound, her air bag would inflate, and the car would fill up with confetti and/or shaving cream. But nothing else would happen. If she has a good sense of humour she will get a kick out of it.
I’m with Kathmandu on this one. I, too, have a great sense of humor, but this would piss me off.
Practical jokes are way overrated.
And you could make a movie about it to boot, I think a good title would be “The Car that Couldn’t Slow Down”…
Don’t use Silly String!
My family did up our car after Mrs. B and I got married with Silly String. Besides being a bitch to get off, it blistered the rubber seals around the windows and windshield. We lost a bit on trade-in value because of it.
When I got married, people wrote on our car with various washable substances, but areas of paint on the old beater was weathered and porous, and some of the lettering was always visible after that. “Good Luck” was written across two different panels, and the “UCK” was on the most weathered car door. Fortunately, it was such an old jalopy, we didn’t care, and it was always good for a laugh.