The Navajo have a tradition around celebrating a baby’s first laugh. Around three months, they watch the baby closely for that first real giggle. The person who has the good fortune of eliciting that first laugh is then responsible for throwing a party.
Two rival churches in Vrontados, Greece shoot tens of thousands of fireworks at each other every Easter in a “Rocket War”. Both churches then claim victory and agree to settle the score next year. The tradition dates back to the Ottoman era, when real cannons were used instead. (source)
Russian cosmonauts pee on the rear bus tire before a space launch. The tradition was started by Yuri Gagarin who did it out of necessity and Russians have since emulated him. Female cosmonauts bring along their urine in vials ready to dump on the tire so they can be part of tradition too.
There was a tradition in Norway, Denmark, particularly on the Faroe Islands, where a girl would present her boyfriend a “willy warmer” (also known as cock sock) to see how seriously he took their relationship. If the gift was rejected, it was seen as evidence that he was not ready for marriage.