Find Tegenungan Waterfall, a huge, gushing waterfall nestled in a green, green valley in Tegenungan Village by driving about 15 minutes' south of Ubud (we wrote directions on the palm of my hand a.k.a. scooter GPS).
You'll pay a small entry fee and you won't have this place all to yourself but it's not very crowded at all. And you're all here to do the same thing anyway - stare in awe at this force of nature and swim and frolic beneath its waters, easy does it, the water crashes down from above with great energy.
On a hot Bali day (and it's always hot in Bali), play like a kid in the cool water, surrounded by mountains and tropical palm trees above.