I did the 3D/2N Ijen and Bromo combined tour for solo travellers. Overall I had a great experience doing this tour even if it was quite expensive. It was my first time to travel alone and everything went well especially thanks to my driver Hadi who was always here to coordinate everything and help me.
The accommodations were simple but clean. I always had hot water which I didn't expect at the beginning.
Regarding specific remarks:
1) The waterfalls were okay. Get prepared to get wet. I used my only raincoat for this activity and needed it for the Mount Bromo tour. When we arrived at the Bromo homestay it was freezing cold and I was still wet from the waterfalls. Fortunately, my raincoat could dry on time. Bring warm clothes.
2) For the Mount Bromo tour, I was ready at 3am as told. However, the jeep driver came around 3:40. It was quite a busy day as it was holidays for local people. Therefore, before arriving at the viewpoint, we were stuck in the traffic and I had to walk more than 2kms alone in the dark in a hurry to arrive at the King Kong hill view point to watch the sunrise (no guide was with me). The jeep driver didn't speak english and it was difficult to understand at what time I should get back to the jeep and how to reach the view point. After the sunrise, we headed to the crater. Our jeep driver gave us only one hour before getting back which was not sufficient time to walk and enjoy the place.
3) We had to wait around 20 minutes for my local Guide at Ijen. Except that, I had an amazing experience in Ijen. I have never seen a place like this, it's so interesting and beautiful. My local guide Eko was really nice and took great pictures 👍.
4) Be prepared to have an intense 3 days, without a lot of sleep: After airport pickup at 10:00 am, we headed straight to the waterfalls and finally reached the homestay in Bromo around 4:00pm. I woke up at 2:30 am for the sunrise and was back at the homestay at 8:30am. After having breakfast and checking out, we drove around 5 hours to reach the next homestay. We arrived at 2:30pm. The tour for Ijen started the same day at 11:30 pm (so almost no sleep). Back from Ijen, the drive to Surabaya took 6 hours.