I'll tell you a few of these ridges from the outside, southeast sector, of Diamond Head. They are all very short, none would take more than 45 minutes round trip, but it is amazing that there are so many complicated ridges in such a small area. I have never seen these little gems, which are good for morning exercises, described anywhere else.
They are generally not truly "hazardous", unless they end up at an obvious cliff and you keep going. The trail surfaces are sometimes not so good, with loose pebbles and sand creating a slippery surface, and so some caution is indeed advised.
I shall assume looking up the crater from the outside in using "left" and "right". The listing goes from left to right. BTW there is trail on the crater rim, which can be walked around the whole way.
1) This is the most prominent ridge coming down from the Leahi peak. Go up Makalei Pl., and on the left there is a fence. Go through the fence and walk up. There is a "Na Laau" monument. Turn right there to go up the ridge. It terminates at a rock face, which seems do-able, but I don't feel like going up. It connects directly to Leahi.
2) To the right of 1). Right next to a blinking light on Diamond Head Road. Go up the trail and turn left. Eventually it reaches a gully where someone seems to be camped out permanently. Backtrack a few feet and cross the gully and go up and around to reach a ridge. This goes up alongside several metal poles and railings. Eventually it comes to a metal railing. One can go on across a narrow dike, but I stopped half-way across. On the farther side seem to be steep rockfaces, which would stop progress.
3) Same as 2), except keep going straight up. There is a narrow dike connecting to a steep rockface. On the right there is a ledge on which one can avoid the dike and keep going some distance into a valley again terminated by a cliff. The ledge is narrow and slippery in places. The ridge connects with 2) below the metal railing.
4) Same as 2), except cross a gully on the right and go up a tiny plateau. Keep going mauka and come to a gentle rockface. I went up partways, but it seems to get steeper and steeper and so I stopped.
5) Right of 4), and just left of the lighthouse. Go up the ridge, and it comes to a steep rock that I managed to climb, but again it just gets steeper and steeper.
6) Directly across from lighthouse. Go straight up and come to a rock which is probably do-able. On the right are several metal wires apparently inviting you to climb up a cliff. I declined the invitation / temptation. Strange, _who_ put up those metal wires?
7) Same as 6), but descend on the right into a gully. It terminates at a rockface, which goes up to a low point of the crater rim. From the top of the crater rim, it looks possible to go down, but I didn't do it. On the left of the gully rockface, one can go up and left, using a metal wire in the process, and come to the same rock with metal wires in 6).
8) Same as 6), but go up the ridge on the right of the gully. It comes to a steep section that one can go around on the right up some bushes. I went up by the bushes and then stopped, but looks like one can go up some more. From the crater rim there is a gentle ridge that goes down, but becomes steep, and it seems to connect, but I didn't attempt the connection.
9) Right of 8), but left of the concrete path. There is a ridge that is becomes knife-edge near the top, but seems do-able. It _may_ be possible to reach the top of the crater rim, but I didn't attempt it.
10) Concrete path: this is the first trail in this list that definitely gets one to the top of the crater rim. Simply go up the concrete path and come to a bunker, which seems to be still in active use. Go around the bunker on the right and continue up a dirt trail. It reaches the top! I was told people in some group _run_ up the path all the way to the crater rim.
11) Right of 10). One can climb on the left, or one can walk up a trail on the right side of the gully on the right, which switchbacks up. The ridge reaches a bunker, and continues to the crater rim. Trails tend to get easier from left to right, and 11) is easier than 10) near the top.
12) From the gully trail in 11), go right rather than left and climb the ridge. It comes to a low cliff. I managed to come _down_ the crater rim, down this low cliff, and come down this trail. Going up the low cliff is probably more difficult.
13) Right of 12), there is a small dirt parking area. Go left up the ridge, which goes all the way up to the crater rim.
14) Right of 13), same small parking area. Go right up the ridge, which goes all the way up to the crater rim.
15) From 14) to the right, access is blocked by houses. Now have to go all the way around D.H. Road, and up the crater access road. There is a parking area with a view just before the tunnel. Go left (while facing the tunnel), and climb up the ridge, which passes by two bunkers in disuse and gets to the crater rim. 13)-15) are all quite easy.
16) Same road, but stop at a turnoff to the right before the parking area. Walk on the road to the end, and then climb up the trail. This trail is quite steep, but I cut steps and it is quite easy now.
17) Beyond the end of the road, there is a fence. Walk along the fence, and the trail becomes increasingly obscure. There is a black lava rock formation on the right. Walk up it, and go straight up. It is overgrown, but is do-able all the way to the top.
18) On that road, backtrack, and there is a right fork. Follow that fork, and it passes by some emergency shelter buildings. Keep going, and there is a faint trail that goes around the crater. Come to a wide, clear spot, and follow the distinct trail all the way to the top.
19) Follow the contour trail, above and beyond the Cannon Club. It crosses a dry gulch, and comes to a wide open area with some kind of equipment (board of water supply stuff?). On the right is a dirt road that leads down to Kepa Street. On the left is a ridge with two metal pipes along it. Follow the metal pipes, which act as very handy hand-holds, all the way to the top.
20) The contour trail becomes wider, and is known as the Na Laau trail. Soon there is open area on the left, leading up to a ridge with metal stakes almost all the way. Beyond the metal stakes, there is a groove in the middle of the ridge. Beyond the groove, there is a moderate rockface that is climbable, and that reaches the top.
21) Another open area, leading up a ridge. It goes to a hidden building that was built by hollowing out part of the mountain. In front there is a flat area on which there is (surprise!) an abandoned railroad track only about 50 feet long. Some people seem to be living here for a long time. Trash is piled knee deep in front of, and inside the building.
The Na Laau trail is coming to an end at the same "Na Laau" monument mentioned in 1).