San Pedro Volcano hands down the best view of Lake Atitlan. You can see his volcano brothers around the Lake and further. The hike is a challenging 4,500 ft of elevation gain (1,400 meters aprox.) and takes about 5 hours Round trip - depending on your level. The top sits about 10,000 ft above sea level (3,000 meters).
The trail starts a few hundred ft above water level. There's a tourism booth on the road towards Santiago where you have to pay the entrance - 100 Quetzales (Guatemala's currency) or 13 USD. Half the price for Guatemalan locals. Price includes a local guide - I recommend going with them as your insurance policy and supporting local guides.
The trail is very clear and you can easily see the signs for the correct route. It's steep. Familiar with the stepmaster? Yes, great for your glutes! There's some rest stations as well as a rope swing - no waivers needed, you're at your own risk like everything in Guatemala.
From the top you can see neighboring volcanoes Toliman and Atitlan to the south. On a clear day you can see Acatenango and Fuego (south east behind the other two mentioned above). To the west you can spot Cerro Quemado.
The hike can be done year round. Best time of the year is mid November to March. Rainy season is from May to October approximately. Bring a light jacket for wind chill and possible rain.