Kirirom National Park
You can really get away from it all at this lush, elevated park a two-hour drive southwest of Phnom Penh. Winding trails lead through pine forests to cascading wet-season waterfalls and cliffs with amazing views of the Cardamom Mountains, and there’s some great mountain biking to be done if you’re feeling adventurous.
From NH4, it's 10km on a sealed road to a small village near the park entrance. From the village you have two choices: the left fork takes you 50m to the park entrance and then 17km up a fairly steep sealed road to the unstaffed Kirirom Information Centre inside the national park; the right fork takes you 10km along the perimeter of the park on a dirt road to Chambok commune, the site of an excellent community-based ecotourism (CBET) program. These are two vastly different experiences, and they are nowhere near each other, so it's recommended to devote a day to each.