Gryffindor: Running at the Whomping Willow and attempting to touch the trunk, legend has it a 6th year grabbed a leaf once.  

Ravenclaw: Every year during the last week of school before break the 7th years attempt to stay awake for the entire week... There have been hallucinations 

HufflePuff: They write small notes of encouragements, then proceed to try to slip them into others pockets without them noticing. A student who was having family issues once emptied his pockets to find 20 different notes from random Hufflepuffs. 

Slytherin: They will hold a conversation and every so often say a word that is complete gibberish but keep going. For every person that doesnt notice they get a point