Like a lot of attractions on the island, we were 2 of only very few people there, which is great for getting photos and not feeling rushed.
Something about being there reminded me of the movie Labyrinth, haha! I've never seen geological features like that so I found it super interesting, and a nice little walk around the perimeter of...
Nothing special, but it's free. You walk around the furnas on a dirt path. It's well laid-out, but only takes about 15 minutes to walk around. It's close to Algar do Carvao, so you can easily fit this in on the way there. The weather was poor the day we went so maybe it's more impressive on a nicer day.
We came here on a great day, weather wise. The hike around the Furnas is nice more for the surrounding scenery than the actual furnas. There aren't geysers or anything, but simply steam coming out of the ground. We did enjoy our walk around them and the signs were informative. Just watch out for all the mud and don't slip.
"Furnas" are a staple of the Azores. You can find a lot more in São Miguel, but this one in Terceira has a special quality. You walk around, see nature, smell the fumes and have lots of laughs when you fall in the mud.