Smaller or larger tuxedo
A friend got married and I, being the best man, decided a humorous practical joke was in order. One of the duties of the best man is to make arrangements for the pick up and return of the groom's tuxedo.
After final fitting, rent an extra coat jacket that is either three or four sizes smaller or larger than the groom's. Explain to the tux shop what you're up to. Pick up the groom's fitted coat, switch with the extra rented coat, and deliver to the groom only when it becomes time to actually get dressed.
The friend of mine wore a 42 long, but the one I provided was a 38 short. Talk about some serious fun! Don't reveal that you know anything as long as possible.
Write on the bottom of shoes
Someone once took a large black ink marker and wrote "Help" on the bottom of the groom's left shoe and "Me" on the bottom of the right shoe. So when he knelt down for his vows, the entire congregation saw it. Of course, this will only work if he must kneel with back to congregation (i.e. Catholic wedding). Make sure you get it so that it is readable with the shoes side by side, left to right, toes toward the floor. Do this far enough in advance so that the paint is dry before the groom wears the shoes to avoid damaging carpets.
Besides "Help Me", other possible message to write on the soles are (with varying degrees of cruelness): Left Shoe (I'm With) Right Shoe (Stupid [pointing arrowhead]); Left Shoe (Quick, Call 911!) Right Shoe (Never Mind, I'm Doomed!)
Cheap plastic rings
A groom buys the engagement ring at a jewelry store. As the groom enters the room, we "have been talking about" the news that says the jewelry store he just went to is being investigated for selling plastic diamonds to unknowledgeable customers. This didn't work too well but maybe if you have better actors in your group it would.
I'll just call my lawyer about this
Another on that I have seen pulled is to have someone speak out at the time the minister asks, "If anyone has good reason why these two should not be married, speak now or forever hold your peace."
They had a pregnant lady stand up and say, "Oh, never mind! I'll just call my lawyer!" It rattled the groom's mother so much that she fainted.
A secret pregnant lover
At the rehearsal dinner for my boss' daughter and son-in-law-to-be, a loud eight-months pregnant teenage girl suddenly appeared at the back of the room screaming ten minutes worth of curses that would befall the groom if he didn't marry the pregnant girl like he promised. It was set up by the boss' wife, and I am told that the groom very nearly burst into tears protesting his innocence.
One way ticket across the country
A groom's friends decided to throw his bachelor party the day before the wedding, and as often happens on these occasions, by the end of the evening, everyone was completely drunk, and none moreso than the guest of honor, who promptly passed out in a corner of the room.
When he woke up, he found himself sitting in an airline seat with no wallet or money or anything on him except a one way ticket to San Francisco. He was worried sick until he finally arrived at San Francisco from New Jersey, where he found a ticket at the courtesy counter back to New Jersey. It seems that one of his prospective in-laws worked for an airline and had some comp airfare to use.
The groom got back to New Jersey barely four hours before the ceremony, and was barely able to stay awake long enough to say "I do".
Return your keys
Before a friend's wedding reception, we passed out keys (blanks) to several girls and one guy. Before some toasts were made, the best man said to the guests, "Now that Jim is married and is no longer available, it is probably a good idea to have any of you girls out there with a key to his apartment to please turn it in now." Then the pre-selected girls (about thirty of them) slowly walked up and handed in their keys as they made bedroom-eyes at him and flirted a bit; some of the girls would turn in not just one key, but six or seven of them. Then the guy walked up, turned in the key, and kissed the groom on the cheek. It's probably not original, but it worked pretty well.
Variation of return your keys
Another twist to this would be to distribute fifteen blank keys to male friends of the bride-to-be and two more blank keys to a guy and a very old lady. Then, during the reception, while people are making toasts, announce to everyone that since the bride is no longer available, any guy with a key to her apartment should turn it in at the tray that has been set up, whereby the fifteen pre-selected men would walk up and turn in their keys and make the same announcement for the groom, whereby both the old lady and the other guy would both walk up with their key.
Do you already have a child?
During the wedding ceremony, when the minister/preacher/priest comes to the part about, "If anyone has any reason why these two people should not marry, speak up now or forever hold your peace..." have this four-to-six year old boy running up the aisle yelling, "Daddy, daddy." I understand from a friend who played this joke on a relative that it took almost an hour to get the wedding started again.
At my cousin's wedding, my dad (who doesn't much care for his nephew's bride) thought it would be funny to flick his cigarette at her back as she walked down the aisle. It got caught in her hair and started to smolder. Her father had to get it out while she cried hysterically. Then he punched my Uncle Raymond, whom he thought had tossed it, right square in the forehead and ended up breaking his own hand. Good wedding.
Obtain access to their getaway car. Fill with balloons. A few extra helium balloons in the trunk with their luggage is a nice touch.
Add some peanuts
If you can get access to their luggage after they have packed, add styrofoam peanuts in whatever nooks and crannies are left.
Impossible to drive away
Jack up the car, put blocks under the axle, then lower the car onto the blocks. When the newlyweds try to make their getaway, watch them rev...and rev...and rev.
Brake wired to the horn
The best man at my friends wedding rigged the horn to sound every time the brake was pressed, they drove half way down the road until they realized what was going on and had to return to fix it.
I deserve to be married
For a small fee, you can get an agency to have a pregnant "ex-girlfriend" appear at the service, in a wedding gown, claiming the groom-to-be the father of her child and demanding *she* should be the one to be married. The one I saw even had a bunch of dead flowers with her!
Who has the ringv?
When the groom asks the best man for the ring, he turns and nervously says he doesn't have it, who then turns to the next groomsman and asks the same question, and so on until the last person turns and grabs a giant box of Cracker Jacks that contained the wedding ring.
The whole church was rolling as the best man and his co-horts had the last laugh. It was truely classic.
Laughing gas in balloons
At a friend's wedding, the bridal party filled their car with balloons - all filled with laughing gas. They put them everywhere, under seats, in the glove box, etc. They popped the balloons, and everyone was relaxed and laughing. But balloons were popping all during the trip of their honeymoon. They said they enjoyed the trick.