This National Park is about an hour and a half outside the city of Puebla (by car). It is also possible to go by bus, though it takes longer.
Popo is an active volcano and no one is allowed on it without special permission. However, you can still hike near it, on Iztaccíhuatl, and enjoy the spectacular views of the smoking crater.
At 5, 220 meters, Iztaccihuatl is quite high up and you can definitely feel the lack of air. It takes about 3 hours to hike up the first part of it. Beyond that section, there is often snow, and to get all the way to its highest point, you need more technical gear.
There are many trails on the two mountains if you want to hike to waterfalls, go for a challenging run or bike ride, have a picnic, etc. There are also rustic cabins should you want to stay overnight.