Early this year, we went on a mini trek to Mt. Pinatubo, a volcano which erupted in 1991 and was one of the largest eruption in the 20th century. It lasted for nine hours and caused major earthquakes. As a result, the entire summit collapsed and created a caldera.
This eruption caused the lost of at least 800 lives and damaged the properties & economy in the area. Before that, it was unremarkable covered in dense forest & supported several thousands of indigenous people called the Aetas.
Today, you can trek up to the crater lake which was formed after the eruption. Water is sulfuric, although swimming is allowed near the banks. Along the way, you can still see the ash formations left behind by the eruption. A small Aeta community still survives in the area, but outside of the main volcano area, the region has nursed its way back to fertile lands.
If you're looking for an adventurous outdoor activity, this is a good day trip option. From Metro Manila, you go by land to Capas, Tarlac. You will then ride a 4x4 along the dusty terrain for about 30 minutes. After which, you will trek the rocky terrain and cross small streams for about 2 hours to get to the crater lake. You do not need to be a hiker to do this, as it has a relatively smooth elevation. At the crater lake, you can have a picnic lunch there and go boating if the weather is good. Upon return to Capas, you can take go for a relaxing mud spa at the Pinatubo Spa Town, before heading back to the city.