Gryffindor: The Quidditch Pitch- it's a great place to practice your flying or to have fun watching a practice. Plus, it's good to scope out the best seats before the matches

Ravenclaw: The Castle- there's so much to explore, and it's nearly impossible to know everything about it. Once in a while, you stumble into a totally new room and it's fun to speculate its purpose

Hufflepuff: The Greenhouses or Courtyards- it's great to hang out with professor Longbottom and help him with the plants, and they courtyards are a great place to sit around and socialize between classes

Slytherin: The Lake- it's pretty quiet and allows for some alone time, and makes a great spot to sit and chat without being bothered by Filch or prefects