If you only have time to do one walk/trail in the Castelli Romani, this is it. In one morning, you can see two picturesque towns, stunning views of the crater and Lake Nemi. Walk through “Bosco Nemorese” the sacred woods of Diana, around Lake Nemi, through historic streets and past fountains and works of art. 
The WALK
Starting from Palazzo Sforza Ceserini in Genzano, take via vecchio corso and wind your way down through narrow alleys in the old town, and take in your first expansive views of Lake.
Take via Perino down to the lake. Once you reach the small beach, continue for another 1.3 km and take the path up on the right towards Nemi. You will see the sign that points to the town.
Once at the top, catch even a higher view of the Lake from Piazza Inammorati, snap a picture of the Fontana della Medusa, and walk the main street of town until your reach the statue of Diana. If you are hungry and want a quick bite, grab a grilled sausage sandwich at the alimentary—they grill them right in front of you, or taste a typical little strawberry tartlet Nemi is famous for. Time 1:30 hour one way.
Sculputers in Nemi by local artist Luciano Mastrolorenzi
You can return the way you came or for a change in scenery, or take the following two ways:
1) Return back to the path you came up, and once back at lake level turn right. Take a drink of cool mineral water running from the aqueducts in Nemi (the fountain is after the ACEA structure).
Fountain found at lake level with natural spring water
Continue walking around the lake; the Temple of Diana can be found off a path leading toward the Agriustrimo. The entrance gate will most likely be locked, but you can catch a glimpse of part of the entrance through the fence.
Back at the lake level, continue around the lake until you pass the Museum of the Two Roman Ships. Once there follow via Diana back up to town, passing a small section of a Roman road. Once at the Piazza Dante Alegieri in town, take a left to return to Palazzo Sforza Cesarini.
2) Return back to the Piazza Inammoriti, and follow via path #511, through the woods above the lake. Check out Fontana di Tempesta and once there change to path #512 which will take you back to Genzano to via delle Piagge. Walking time 1:45 one way. Distance: Round trip 15 km, Time: 3.5 hours
Fontana di Tempesta
How to arrive:
Cotral buses from Metro Station Anagnina arrive to the main piazza in Genzano di Roma.
Parking: There is a parking lot in front of Palazzo Sforza Cesarini, Genzano di Roma
What/where to eat: Taste typical products such as the Nemi’s strawberry tarlets, or Genzano’s breads and pizzas.
Azzocchi, Corso Don Giovanni Minzoni, 21, Genzano to taste all their delicious cured meats and cheeses, and grilled meats.
Level: Moderate.
Trails mentioned: Sentieri 511B & 512 & 515