The Shihmen Reservoir (石門水庫) is a major dam and reservoir about 30 kilometers south of central Taoyuan, set in dramatic gorge country where the Dahan River cuts through the foothills of the Central Mountain Range. The dam itself is an impressive piece of mid-20th century engineering — completed in 1964, it was one of Taiwan's major infrastructure projects — and the views from the dam crest over the green water below are genuinely striking.
The surrounding park has walking trails, a popular freshwater fish restaurant strip (the local shrimp and eel are excellent), and boat tours of the reservoir. In autumn and spring the hillside scenery is at its best. It's about a 45-minute drive from Taoyuan, and most visitors combine it with Daxi Old Street and the Chiang Kai-shek memorial sites to the north.