How to get there:
This excellent web-site on climbing Indonesia peaks should always be consulted.
This peak is located near the town of Batu just outside the city of Malang. I discovered a really nice trail, that I highly recommend. Drive about 15 kilometer (as the crow flies) up the valley directly north of Batu. You will see Arjuno on your right hand side. Near the col, you pass through a small village. Continue to location S07.74924, E112.53480, elevation near 1740 meter. This is close to the main col. Turn right here onto a smaller road that serves the farm above, this road still has a hard surface. Ascend to the higher plateau with farm land. The road now turns into dirt and becomes rougher. My taxi stopped at a local road fork at location S07.75553, E112.54299, elevation about 1900 meter. Any car should easily make it here. The road continues uphill to location S07.75460, E112.54868, elevation near 2000 meter. This is the trailhead.
Here is a summary and quick reference to the climbs on this trip.
The farm road ends near a small building. Look right for some trail marking ribbons. You should descend slightly (right) in order to get to the lower right corner of the farmed area. The trail then ascends next to this upper piece of farm-land on its right side (looking uphill). Shortly, you arrive at a small clearing where another trail (coming from below) merges. Continue uphill along a clear trail. Follow this as it gently ascends, a very nice change from what are often very steep uphill climbs in Indonesia.
After a bit more than an hour (from the trailhead) you should arrive at a clearing, location S07.74617, E112.56953, elevation about 2580 meter. Take the trail that climbs right from here. You will soon pass a sign saying Puncak Arjuno, confirming that you are on the right track. This trail continues to a very nice col at location S07.74605, E112.57942, elevation about 2900 meter. The peak on your right is fairly low, but prevents the view of Arjuno. The trail now contours around this hill, loosing about 100 meter of elevation in the process. You arrive at a small meadow and finally see the peak in front of you. Ascend to the ridge and follow this to a first summit. The last part of this ridge is almost horizontal with big drops on either side.
Arriving at the summit, you will immediately notice that there is a second summit further onwards connected by a shallow saddle, perhaps 30 meter deep. GPS measurements cannot determine which is higher, they seem to be within one meter, so one should really visit both. Fortuneately, this is a short (10 minutes) walk.
I climbed this the day after a pretty hard day climbing Butak. There was one trip report on Peakbagger and I decided to try that route since it looked short and pretty straightforward. My taxi driver that had waited longer than expected for me to return yesterday, was happy to get a new assignment.
We drove up to the temple/monastry called Pura Luhur Giri Arjuno, passed through a big gate and stopped to talk to some local farm workers. They informed my driver that the gate would close at noon and that they (at least this is what he told me) did not want anybody hiking the trails there. Appearently, guide Sam had made some special arrangements when the Peakbagger reported ascent had taken place.
I then switched to plan B, ascending the peak from the col at the upper end of the main valley. This turned out to be a very fortunate decision. I initially thought that I would have to hike from the main Hwy. however, it looked easy to drive at least part way up the first hill. My taxi did not object and soon we were at 1900 meter, a pretty high trailhead. Here, the road turned a bit rough and it made sense for the taxi to stop.
I started hiking at 0815. To my delight, the trail was really nice and not just a steep uphill. It did climb, but very gently. I carried on and noticed a sign saying Puncak Aruno, always nice to know. I crossed the nice saddle and then contoured around the hillside that obstructed any views of Arjuno. Finally, the peak came into view. Still, quite a bit of vertical ascent. This part of the route was by far the steepest, still pretty good. I arrived at the (first) summit exactly at 1200, so 3:45 up.
I looked across to the second summit and headed over there after about 5 minutes. As it turned out, these two summits are indeed very close in elevation. A GPS is not able to tell any difference. The second summit is more rocky, a couple of large boulders competing for being highest. I next returned to the first summit, enjoyed the view as well as some water to drink before descending.
I left the summit at 1235 and had a good hike back down to my taxi (waiting), arriving there after 2:45. Perhaps surprisingly, I did not see anybody on this trail. Quite different from the high peaks further west where quite a few people always would be present.