In this picture you can see the Fuego Volcano, which means Fire in English for no Reason, the volcano is still active. Only expert climbers can make it to its top.
If you want to climb an active volcano, I recommend Volcano Pacaya, which lies near Guatemala City, thus it is not visible form Antigua. You will find several agencies that organize tours to the volcano. Everyday there are two tours, one departs first thing in the morning, and the other around 14h. The path is quite steep, but if you are fit, you can make it to the top in two-three hours. At the top you will be able to see the volcano's glowing lava.