Take comprehensive travel insurance with medical evacuation cover — serious cases in Bali are usually flown to Singapore, and hospitals expect payment or a guarantee up front.
Read the motorcycle clause before hiring a scooter. Most policies only cover scooter injuries if you hold the correct licence plus an international driving permit, and wore a helmet.
Bali's dry season is April to October. Chinese New Year, the early-May holiday, the summer break and National Day in early October are the peak windows; April, May, September and late October offer better value.