You've unlocked the Thestral Mount, the Dark Arts Cosmetic Set, and the Dark Arts Battle Arena. New mounts are accessible in the Gear Menu after the default Hippogriff is unlocked in the main questline. Access cosmetics in the Gear Menu by placing the cursor over gear of the same type and selecting the Change Appearance option. The Battle Arena is accessible in the Forbidden Forest once you progress the main questline far enough to leave Hogwarts freely.