When you have collected ALL 200 Gold bricks, go to the cinema room in the Leaky Cauldron and you will see a giant pile of Gold bricks. If you build the sculpture it will constantly keep spawning Gold LEGO studs as long as you are in the room or the hallway.
Where to find Luna (purple jacket) ? -> character token
In the level 'Lovegoods garden'. When you knocked on the door and you are inside, Xenophilius go upstairs. Below there are somewhere pots and pans. Shoot it, they form a drum kit. Shoot the biggest drum after it stops playing. Luna's token appears