The first time I saw volcanic activity was in Rotorua[0], in New Zealand... I think the big volcano there is dormant but because it sits on top of a huge caldera, it still has plenty of volcanic activity, including boiling mud and geysers! It's an amazing place to visit. You can feel the warmth of the rocks and near the geyser, there are water ponds nearly as hot as boiling (incredibly clear water too). Seeing the geyser explode is the highlight of the visit, of course... But just seeing the boiling mud (which you can see everytwhere in town, basically) and hearing the noise of boiling liquid under your feet is an amazing feeling.

I was young and stayed in the local backpacker which had a naturally-warm water pool!

As you approach the city, you feel a very strong smell of sulphur (a smell described usually as rotten eggs), but as you stay in the town you more or less get used to the smell and stop feeling it... it's not as bad as it sounds.

Highly recommend a visit if you're around Australia or the South Pacific.

One day I want to visit Iceland as well, it looks amazing.

[0] https://en.wikipedia.org/wiki/Rotorua