With temperatures that vary between 22 and 32 °C, Indonesia is a pleasant country to visit year round, even during the rainy season from November to April.
Don't overload your suitcase. One or two shorts, a pair of trousers, one or two dresses, a few T-shirts and one or two swimsuits are all you need for a holiday in Indonesia. In addition, it is very easy to have your clothes washed in most hotels for less than a pound. However, don't forget to bring a good sunscreen, sunglasses and a hat: the sun hits hard in Indonesia. It is advisable to take a pareo or a sarong to visit the temples, to cover your shoulders and legs if you are wearing shorts. But you can always buy a sarong on the spot.
In the tourist areas of Indonesia, you can you dress as you like without risk of offending the locals. However, be careful of what you wear in more remote regions. Women should opt for trousers or a long skirt and cover their shoulders. Men should leave their tank tops at home. This kind of t-shirt is considered as nightwear in Indonesia... If you go swimming in a remote area, observe the locals. If they swim fully dressed, do the same. You will be dry in 30 minutes and will avoid offending the sensitivities of the Indonesians.
If you are going for a trek on one of the many volcanoes of Indonesia, take a pair of hiking shoes, a fleece and a windbreaker. For example, during an ascent of the Rinjani, you will sleep at over 2,000 metres of altitude where the temperatures drop to around 0 °C (or even less).
Finally, take a little first-aid kit with the essential medicines: anti-diarrhoea, antihistamines, antibiotics and anti-malarial treatment. "Tourista" is common everywhere, and the risk of malaria is present throughout the country, except in Bali.