The best water on the island is two hours north of where you are staying. That drive is the only reason it is still quiet.
There are two kinds of waterfall in Bali. There are the ones near Ubud — Tegenungan, Tibumana, Suwat — which are twenty minutes from a flat white and busy by nine in the morning. And there are the ones in the north, around Singaraja and Munduk, which take two hours to reach and are genuinely spectacular.
Sekumpul and Banyumala are the two worth planning a day around. Aling-Aling has the jumps and the slides if that is your thing, and a guide is compulsory for the upper falls whether or not you want one.
Leave at six. Every one of these is doable as a day trip from the south, and almost none of them is pleasant after nine.
20 places, ordered by rating weighted against how many people have actually been.