During the wet season (late November, December - April), roads can be blocked and trekking routes closed for safety reasons, so it’s best to double check how much this will affect your chosen itinerary.
The shoulder season (March, April, May, October and November) is fairly unpredictable weather-wise, but both prices and crowds drop and temperatures are cool making it a great time for trekking.
During December, January and February rain makes areas such as Sumatra and Flores virtually inaccessible due to closed ferries and washed out roads. There are plenty of other places to see at this time though - with Bali and Kalimantan enjoying much more stable climates.
If you're heading to the Mount Rinjani National Park on Lombok to see the infamous volcano, plan your Indonesia vacation outside of January – March, when the park closes.
If you’re heading to Java, avoid the end of July – these are the final days of Indonesia’s Lebaran celebrations and transport is utter chaos.
Indonesians often choose to take vacations at the end of Ramadan (usually around June or July, but flexible, so best to check according to your chosen year of travel) which can mean more crowds and higher prices.
September in Indonesia brings the favourable weather conditions of August, but without the crowds and high season prices, making it a brilliant time to visit.