If you want to see the Blue Fire and the sunrise, you should set off early. The gates open at 2:00 a.m., and you can begin the climb up Ijen. It's about 4 km to the top and 600 meters in elevation. The path is wide and well-maintained, with some steep climbs. You'll definitely need a headlamp. Then you can descend into the crater to see the natural phenomenon known as the Blue Fire, which is caused by the combustion of sulfur gases. The path is rocky and requires some surefootedness—but I found it manageable in sneakers. The fire wasn't big when I was there, but it was still fascinating!
You definitely need a mask, as it smells strongly of sulfur down there! Masks and headlamps are available for rent.
Afterward, you climb back up and can watch the sunrise. You need a bit of luck, as fog often rises from the crater, and you can't see much.
Truly beautiful, BUT it's a mass event! Many of those who climb up, and especially those who descend into the crater, don't have the fitness for it, and especially not the surefootedness. So, you have to set off right away at two o'clock! People are also crowding around the blue fire.