I'm sure everyone has head of the great Bali beaches. Pristine white and black sand beaches touched by the blue/green waters, and drenched in sunshine. Don't believe it. Nothing but trash on the beaches and in the water. Yes, tourist contribute a small part but natives could care less about where they throw their trash. One unfortunate thing is that while the hotels do try to keep the beaches clean, the current refuses to cooperate and brings in more trash from areas in Java. Totally disgraceful. While the local media reports on it, the Tourist Ministry keeps trying to sell you "Wonderful Indonesia."
Other places in Indonesia are just as full of trash or even worse. All contributed to the natives habit of every weekend packing up the families, heading to the beaches or public areas, cooking on the beach, and in the end, leaving all their trash scattered where they sat. Of course, they throw it out of their car windows and have no reservations about tossing a trash bag from home directly into the ocean. Wherever they are near.
While the main tourist locations are trash, there are still some nice beaches. Lombok has nice beaches for now but they are pushing the area for tourist now. The beaches are OK but to get to them you have to go through piles and piles of trash and garbage because they have no way of getting rid of it off the island. Flores Island has some of the most beautiful beaches in Indonesia but lacks any modern or close to modern facilities for hotels and sanitary food outlets. Belitung off of East Sumatra in the Rial Isles has beautiful, well maintained beaches. Packed during the weekend but relatively empty during the week. Mandiri Beach between Tanjung Senang and Kuri in Lampung, Sumatra is gorgeous. During the week you can walk North or South a few kilometers and never see a soul. One entrance only towards the center. This area also has 2+ meter surf in the season from March to November and around 1 meter surf in the off season.
Not to leave out one area. Ache has great beaches in North Sumatra but unfortunately, if you were something that would be normal swim wear in the West you could get arrested. Sharia law rules there and people i know avoid it like a plague. Bikinis are pretty much since Indonesia is a predominately Muslim nation. The ladies can get away with being topless on a lounge chair on Bali's beaches even though it is illegal. Bali is Hindu and know where their finances come from. Of course, you will get the ooglers and the "hey, I'm a stud" locals around you. The rest of Indonesia, guaranteed someone would report you to the police and they would come if for no other reason to collect some cash off of you. Any beach is best during the week days except Bali. Always crowded.