Peak season: July and August are the two months when there is a lot of tourist activity across Indonesia. Prices might be on the higher side so it is advisable to have advance bookings.
Shoulder season: May, June and September are the months when the weather is good in Java, Bali and Lombok. Commuting may not be a problem as there are less number of tourists compared to the peak season.
Off-season: October to April is the period when you can get cheap deals on packages except during the Christmas season. The weather tends to be dry which is ideal for diving in Maluku and Papua.