Vulcano at Una Una (about 40 km from us, around 1 - 1,5 h with our speed boat) erupted last time in the 80's. Still today it's active, and can be easily experienced by hiking up! There's two routes, one leading up from the east and in the top is a vulcanic lake. The more exiting one is on the north side of Una Una: This is where the nature is showing its full power.
There's two ways of starting the trip: either you drive with a boat to the river mounding or you drive from one of resorts at Una Una or from the main village with a motorbike to the starting point. If you take the bike option, make sure it hasn't been raining the whole morning. Otherwise you're more stuck in the mud than on the way...
After you reach the river, you will start hiking. It takes only about one hour to reach the top, and it's actually quite an easy hike following a river path. Just make sure you watch where you put your feet, as the closer you are to the top, the hotter the stream gets. On the way there is an amazingly beautiful waterfall, and it has almost boiling temperatures. And it's salty!
When you get to the crater (or at least it looks like one), you'll find boiling spots of water (remember to bring eggs to boil), a big geysir and pretty much water and steam huffing and puffing everywhere. Not very good for photos, better you enjoy with your eyes :)