The view from the top of the caldera was unusually clear yesterday. In case you're up here, the rest stop on the 339 right before it T intersects with the 45 sells awesome takoyaki.
This waterfall can be found on the Oike pond hike, in Kuju. From Aso take the Yamanami highway (the 11) towards Beppu, and about 20 minutes after you pass Maki no To (the highest point on the Yamanami from which people start their ascent on Kuju), follow the signs towards Oike pond. The pheasants were out in the clear cut woods on the way there, an intense concentration of iridescent colors among the desolation.
I can't believe I screwed up so badly. I took so many good pictures at the Kuju Hana Koen, and then erased them all. I also highly suggest this place right now, as the summer flowers are in bloom. However, my favorite time to visit is in the Fall when the Cosmos are in bloom. To get there, take the Yamanami towards Beppu, and follow the 442 towards the flower park (take a right).