For my friend's 30th birthday we threw her a "Honk 3 Times, She's 30" party.   For the invitations, we sent out a homemade card with an old air-horn on the front saying "Honk 3 Times, She's 30".  Inside we gave instructions for everyone to drive past her house at least once and honk three times. And then to come to the party at 12:00, but instead of presents, bring a dish of food to share. On the day of her birthday, we drove by her house at about 8:30 AM and snuck a sign up in her front yard.  She lives on a pretty heavily trafficked street, so lots of people were honking. We showed up to set up the party at about 10:30, and while we were setting up she kept going to the window to see what was going on. She commented on how much honking there had been today.  For lunch we had about 10 people who showed up, each with a different dish, and we toasted her and then before we left, we took her out and showed her the sign, which was hidden from view of her window by a hedge. Luckily, she was a good sport and didn't retaliate too badly.