Be careful guys...I have seen more than one set of "best highschool friends" play this game, and they ended up enemies for life because it eventually went to far. Somebody finally posts something, or says something, that just crosses the line and it's all over.
But, with that said, some of my adult jokes are this...
1) Find a time when her family is away from home. Take yellow "Police" or "Crime Scene" tape and run it all the way around their house, even block off the drive. Then, in the grass, spray paint the outline of a body. Sit back and watch their expression when they come home.
2) Put a "For Sale" sign in there front yard with their phone # on it. Put another one out by the highway at the entrance to their neighborhood.
3) Find out exactly what kind of car they drive, and list it in the local paper with her cell phone #, and list it dirt cheap so there will be about 500 calls a day.
...I could go on and on....