The volcano Poas in Costa Rica is the easiest accessible active volcano crater I have seen so far. You basically park the car, walk there some 10 minutes, and you are at the crater! It is one of the largest active craters in the world! it was truly stunning. Throughout the year, its color varies from blue to green. It was very green (and calm) when I visited it! Besides, Poás is one of the largest craters in the world.
That was in the first days of 2007. I went back to Costa Rica in 2018 and it was at that moment closed due to an eruption. As I’m writing this, it seems open to visitors again, but tickets need to be purchased before hand.
My pictures of the volcano Poas: