Indonesia has lots of very beautiful beaches, Bali and Lombok are my favourite. Oh i miss it, how you can lay and relax under the sun on very white sand. Beautiful image indeed!
Anyway if you are from other countries, you would not know how the weather there. It will be pain if you come, and plan to expect sun, but only rains will come.
Because we don’t have winter (i hate winter!) we only have rainy seasons and summer the whole year, even in rainy seasons, the temperature won’t go below 15 degrees celcius, still warm.
Due to Global warming, the weather in Indonesia also can be changed unexpectedly just like in Europe.
The rainy seasons can come on May, where it should be hot summer, but as mentioned above, it will be still warm to stay without t-shirt (for males) 
The best time or months would be between May to August, but common problem, you will have to stay with thousand other tourists in these favourite months.
I recommend you try Lombok beaches also, as it doesn’t have as much as tourists like in Bali.
Happy travelling!