When we visited Antigua we hiked Volcan de Fuego and Volcan Acatanengo. It is an overnight hike and we used the outfitter linked here with a guide named Elvin. It was an incredible experience and we recommend it to anyone who enjoys hiking!
Lake Atitlan is an ancient lake surrounded by volcanoes. It is a ~3hr bus ride from Antigua (https://guatego.com/Guatemala/Antigua_Guatemala-to-Atitlan_Lake/). The bus will take you to Panajachel, where you can explore or take a boat to one of the many villages surrounding the lake.
Browse the local market to get a snack and drink or pick up some traditional handicrafts and Guatemalan textiles.