Hi Derek, Costa Rica has tons of cool volcanoes, it’s definitely hard to pick just one. Poas and Irazu are both crater volcanoes. Poas has a cool blue crater lake- you can read more about it in this post: https://www.twoweeksincostarica.com/racing-the-clouds-poas-volcano/. We ended up not being able to see the crater at all when we visited, but if you’re coming in December, it shouldn’t be as cloudy. Poas is a gorgeous area- slightly cooler and very green- and a place you could easily spend a few days. La Paz Waterfall Gardens, Catarata del Toro (awesome waterfall- more here https://www.twoweeksincostarica.com/catarata-del-toro/), Doka coffee tour, etc. are all in the area.
Irazu is on the other side of San Jose, near Cartago. Its crater lake is sometimes dried up and can also be clouded over, plus there isn’t a ton else to do in that area, so I’d recommend Poas over Irazu.
Arenal is definitely popular like you said, but for good reason. It’s huge and really cool to see even though it’s not erupting anymore. There is also a lot of great hiking/opp for nature exploration around Arenal…something that you won’t find with Poas and Irazu, which don’t have many trails. Arenal 1968 trails, Arenal Volcano National Park, etc. are all near the volcano, plus there are a ton of other activities in the area. Check out our La Fortuna post for more info on activities, hotels, etc. in the area: https://www.twoweeksincostarica.com/la-fortuna-what-to-expect/
One idea would be to do a loop for your nature exploration week, first going to Poas for a few days then onto Arenal (this is a gorgeous drive through the mountains).
One more option that comes to mind is Rincon de la Vieja National Park in Guanacaste. Rincon creates geothermal energy and you can see bubbling mud pots and steam vents at the park. There’s a fairly extension trail system too, and a waterfall which should be flowing in Dec. Horseback riding, zip lining, mud baths, and other activities are available in the area. And not sure what you were thinking for which beaches to visit, but Rincon would be easy to pair with the beaches of Guanacaste.
Best of luck planning your trip!