Songkran (Thailand): Every year from April 13 to 15, Thais welcome the New Year (according to their calendar). Because Songkran falls in the middle of the hottest season in an already hot country, how do you think people celebrate? Every Thai heads out into the streets with water guns and buckets of ice water -- sometimes laced with talcum powder, just to add to the mess -- and spends the next 3 days soaking one another -- and you. Foreigners are especially favorite targets. Don't get mad: Arm thyself! Water bazookas are on sale everywhere. Have a ball!
Dragon Boat Races (Laos): Celebrating the end of Buddhist Lent, dragon boat races are held in every riverside town in Laos (and that's most towns, really). The races are exciting, the betting is frenzied, and there's always a small carnival with handmade rides and the standard rigged games of skill.
That Luang Festival (Vientiane, Laos): In early November, thousands of Buddhist followers from all over the country, and even a few neighboring countries, converge on the spectacular That Luang temple in Vientiane. There are alms-giving ceremonies and flower processions, and then the whole affair dissolves into a carnival that stretches over several days.
Chinese New Year (Singapore): If you're in Southeast Asia around the end of January or the beginning of February, hop up to Hong Kong or down to Singapore for the festivities. It's a 3-day party, with parades (complete with dragons and stilt walkers) and fireworks.
Thaipusam (Singapore & Malaysia): Around the end of January and the beginning of February, Hindus celebrate Thaipusam. Men give thanks for prayers answered by carrying kavadis, huge steel racks attached to their bodies with skewers piercing the skin. Cheeks are pierced, and fruits are hung from the skin using sharp hooks. A parade of devotees carry these things in a deep trance -- and the next day they wake up virtually unharmed.