Beach clean-ups are a big thing in Bali! Try following groups like Trash Hero Canggu or One Island One Voice on social media to see when their next clean-up is, or for news of any events happening during your trip.
I've also been exploring going back to North Bali for a trip, and came across Pemutaran, a town on the north coast that is home of one of the most successful Bio Rock programmes in the world - planting rocks underwater to let coral regenerate. It was a brainchild of the local community, which also organises beach clean-ups on a regular basis. I'll definitely check it out if I ever make a trip there and report back!