Wet season in Java, Bali and Lombok (and Kalimantan flowers).
Dry season (best for diving) in Maluku and Papua.
Easy to find deals and you can travel with little advance booking (except at Christmas and New Year).
(May, Jun & Sep)
Dry season outside Maluku and Papua.
Best weather in Java, Bali and Lombok (dry, not so humid).
You can travel more spontaneously.
(Jul & Aug)
Tourist numbers surge across Indonesia, from Bali to Sulawesi and beyond.
Room rates can spike by 50%.
Dry season except in Maluku and Papua, which are rainy.