Thai New Year transforms Bangkok and Chiang Mai into the world's largest water fight. Water symbolizes washing away bad luck for the new year — the original temple ceremony has evolved into three days of street battles with water guns, trucks with mounted hoses, and entire neighbourhoods turned into soaking warfare zones. Silom Road in Bangkok and the Old City moat in Chiang Mai are the epicentres. It's 40 degrees, everyone is laughing, and staying dry is not an option. Chiang Mai is considered the most atmospheric — the moat fills with floating flower offerings and traditional parades alongside the chaos.
Protect electronics obsessively. Chiang Mai is more traditional and beautiful; Bangkok is more intense and urban. Both are worth doing. Don't plan to go anywhere fast — the city slows completely.