“Very impressive, if you... go around!”
The volcano itself if impressive, but... japanese people is paranoid about safety, so you should make some efforts to have pleasure there.
If slightest fume goes to observatory point, they immediately close the entry and try to stop you, if you go by foot. But NOBODY speak English, not one word... You should show them, that you want to go not to the observatory point, but to go around, and they could allow.
You should go up by foot up to the fork in the road - left one leads to observatory, right one - around crater. First it's wooden path, and after - just trekking.
It's very easy to go around crater and it takes 2 hours if not hurry to make almost full circle and reach another observatory. From this second observatory the road goes down to another cable car, but I don't recommend to go there, because no bus goes from there.
The way back to the main cable car takes around 1.5 hour. If to go fast, you can go there and back in 2.5 hours only.
Take a mask - just in case. Sometimes you feel the slightest sour smell from the crater, but it's not dangerous. If it strong - put a mask.
From the main observatory it's excellent view down in the crater with it green lake which consist of hydrochloric acid, sulfuric acid and aluminum.
The cable car costs about 10-15$, but by foot it's more interesting and only 10 minutes to go along the road. Entry on the path - on the right if you stay faced to entry in cable building.
Many times japanese paranoid staff will try to prohibit you to go here or there. Just demand that they will speak English. They could be aggressive if you only make 1 step aside. I think that this is demeaning and not affordable. Protect yourself from this aggression with demanding of speaking English.
The buses goes to Aso volcano every hour from the Kumamoto (from bus station, from Tenmonkan) - see schedule. The price for forward-back bus-ticket is around 25$
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.