There are atleast 3, and sometimes they are used as classrooms.

The Whomping Willow is a very valuable, very violent tree planted alone in the middle of the school grounds. It was planted the same year that Lupin arrived at Hogwarts to disguise the opening to a secret passage from Hogwarts to the Shrieking Shack. Lupin would go through a hole in the Willow's roots every month to transform into a werewolf in the Shack where he couldn't hurt anyone. The Willow was so dangerous that it kept other people from entering the passage and encountering the werewolf.

Located to the south of the castle, which stands on a cliff overlooking the water. The lake is approximately a half-mile in diameter. Also various creatures live in the lake.

There is a wide flat lawn opposite the castle from the forest and Hagrid's hut where the first-years took their first flying lesson, and sloping lawns lead down toward the forest and border the lake.