Leave their car alone:

Every old black and white movie that had a wedding in it, featured the car leaving the church with a "Just Married" sign and a string of cans tied to the back.  It was a good-natured way of having some fun and wishing the new couple well.  I can remember different times when I saw cars similarly adorned going down the road.  My wife and I would honk and wish them a happy wedding day.
I've been doing weddings since 1975.  Things have gone way beyond a few tin-cans.

• I've seen cars jacked up and put on stands so when the couple gets in the wheels turn but they don't go anywhere.
• When my younger son got married it took me about four hours to clean up his car.  I think it took my daughter-in-law's dad even longer to clean her hers up.  The newly weds had wisely planned to leave in a borrowed vehicle, which arrived at the church at just the right time.
• When that son was just a kid, he was offered an impressive bribe to let the "buzzardly friends" into my garage where the grooms pickup truck was hidden.  It turned out that it was only a decoy anyhow.
• I've heard of paint jobs on cars being ruined.
• Some folk's cars are essentially undriveable for a few weeks, while the stench from whatever wears off.  Sometimes it doesn't.
• I could say more, but I don't want to give the buzzardly crowd any new ideas.

Trust me, I know more about this than the average buzzardly friend.  Wedding days are stressful, especially for brides.  Let's not add to it.

Granted many a groom shows up at a wedding having built up an incredible debt of bad matrimonial Karma.  He has messed up many a car.  He is one of the buzzardly friends that I warn couples about.  To all the other buzzardly friends, figure out some other way to get even with him, leave the young lady out of it.  If she married a guy who did all that to other people, she probably already has enough issues to deal with.
Or, here is a really novel thought.  Show some mercy.

To all of those who do not fall in the buzzardly friend category.  Thanks.  Don't join their ranks.  Dare I say it?  Do what you can to curb their enthusiasm for automotive mayhem.

I figure this has as much of chance of gaining a following as my campaign for President on the Curmudgeon ticket, but I feel better.

If you must do something to the new couple's car, put a $100 in the console.  Believe it or not, it is fun to be nice.