I haven't been on those windward Kohala yet - just Kohala's boggy summit area. One of the "ditch trails" was wiped out by landslides during the little earthquake a while back. They're still haggling over whether or how to fix the flumes.
I live in Hilo, so generally I explore on the Eastern Saddle off of Saddle Road, or off of upper Stainback Highway near the Kulani prison. Sometimes on the upper slopes of Mauna Kea or Mauna Loa, or in the Ka'u Desert. Keanakolu Road is too much for my little car, so I don't get to the Eastern and Northern slopes of Mauna Kea. I just pick a jeep trail or hunter trail and start exploring, or head up/down the lava fields visiting all the kipukas. I'm mostly interested in the native flora and fauna, so I go where the rare stuff lives. One of the best areas for native birds is on the Eastern saddle near the end of Powerline Road, and at the beginning and end of Pu'u O'o Trail. (Named after the Pu'u O'o on Mauna Kea, not the other Pu'u O'o on Kilauea) The Eastern Saddle is absolutely wonderful when the weather is cooperative, and the large pahoehoe lava fields make exploration easy. One of the best areas for native wet forest plants is the rainforests at the upper end of Stainback Highway. There are many abandoned roads up there for rainforest access, and rough hunter and biologist trails everywhere in the forests. Some people get lost in those forests for days...or the short last few days of their lives. The slopes of Mauna Kea and the Ka'u Desert are good for rare subalpine and dry forest plants. I have years of exploration to do up there before I've seen it all.
Favorite area on Powerline Road:
http://maps.google.com/?ie=UTF8&ll=1...18046&t=h&z=16
Favorite area on Pu'u O'o Trail:
http://maps.google.com/?ie=UTF8&ll=1...18046&t=h&z=16
Upper Stainback Highway:
http://maps.google.com/?ie=UTF8&ll=1...36092&t=h&z=15
Western slope of Mauna Kea:
http://maps.google.com/?ie=UTF8&ll=1...36092&t=h&z=15
Southern slope of Mauna Kea:
http://maps.google.com/?ie=UTF8&ll=1...36092&t=h&z=15
and many many miles of bushwacking around those areas and many others. Almost all of that is Forest Reserve and Natural Area Reserve lands.
No waterfalls or ocean on any of this.