Agua is the easiest and by far the most popular of Guatemala’s big cones to climb, and on Saturday nights dozens of people spend the night at the summit. It’s an exciting ascent with a fantastic view to reward you at the top. The trail starts in Santa María de Jesús: it’s a fairly simple climb on a clear (often garbage-strewn) path, taking five to six hours, and the peak, at 3766m, is always cold at night. There is shelter (though not always room) in a small chapel at the summit, and the views certainly make it worth the struggle.
As there have been (occasional) robberies reported on the outskirts of Santa María it’s best to team up with an Antigua adventure sports company (see Tourist crime in Antigua) and not attempt the hike on your own.