Of course, it is stricly forbiden to go to the crater lake... this is just why you have to do it ! Beware the toxic fumes which are really uncomfortable (It hurt my lungs for 2 days). I don't know how the worker can bear it.
This one is not as picturesque, though very colourful. Turquoise crater lake and bright yellow sulphur, carried back to the real world by brave (and underpaid) Indonesians. It's a short hike to climb, I've seen 70 y-o on the trail, it's no big deal, people!