San Pedro Volcano became a national park in 2006, so it is hoped that, as was the case with Pacaya Volcano National Park [1], its newly protected status will result in greater police presence and an end to the robberies that frequently happen near the summit. For now, check with locals before heading up the volcano. Under no circumstances should you attempt this hike alone. Always go with a local guide. There is a visitors center at the trailhead, which is just off the road to Santiago.
The hike is fairly strenuous, as the trail runs straight up the mountain with very little in the way of switchbacks. It takes about 4–5 hours to reach the summit, which is still very much covered in thick cloud forest. There’s a small gap in the trees at the top from which there are views of Santiago and the lake. Start your hike early in the day to avoid the midday heat and the clouds that typically gather at the summit of the lake’s volcanoes in the afternoon.
Links:
[1] http://www.moon.com/destinations/guatemala/guatemala-city/near-guatemala-city/pacaya-volcano-national-park