You're looking for a camouflaged pill bottle hanging from a tree. This area looked pretty swampy when it was placed in the winter months. It might be wetter during the summer.
Be stealthy there's sometimes a lot of muggles in this area. My brother used to play football in the field that leads to the cache. When he played baseball they had parties in the pavilion next to the field. The playground that we grew up with was 10x better than the one that replaced it. Hide and seek was more fun with all the hiding places the castle used to have. It even had a "telecom" that you could speak to your friend from the other side of the castle. It worked like magic. There was a slide that took you to a dinosaur world (wall painted with dinosaurs) with a tunnel to climb out of. As I did some research, it turned out that the creators of this playground asked the kids in all the schools what they wanted in their dream playground. No wonder they did an amazing job.