Now that you have the Room of Requirement to yourself, decorate it to make it your own personal lounge.
Ditch school one day, got to the Ministry, and start poking random officials.
Apparate in and out of the Burrow to annoy Mrs. Weasley
Go to St. Mungo's and roll patients' gurneys down the corridors (What? They needed fresh air)
Take the enchanted flying Ford out for a joy ride
Hitchhike down a busy Diagon Alley, bump into strangers, and don't apologize
Let Fluffy, from the 3rd corridor, the basilisk from the Chamber of Secrets, and a mountain troll loose in Hogwarts. When they ask you who did it, say Hagrid gave you explicit orders.
Camp out in the Forbidden Forest and go swimming in the Black Lake.
Convince a couple of naive first-years into making a tree house on the Whomping Willow. Show it to Snape.
Have an exclusive toga party in the Room of Requirement you renovated. Say Pansy is the host.
