Do I need a driver's license to rent a motorcycle in Chiang Mai?

No, just a copy of your passport and a deposit of 1000 baht. Ordinary motorcycles cost 180 baht a day, which should be the lowest. But be careful when refueling. Don't refuel at the gas station downtown. It's too expensive. If you refuel at a gas station outside the city, 50 baht a day is enough. Another thing to note is that the traffic rules in Thailand are to go left, not right, or you will be despised.