1) Braid a manner of random stuff in his hair (forks, beads, compasses, whatever you like) while he's asleep
2) After completing number 1, wait until he wakes up and say, "My, my, Professor, that roguish Captain Jack Sparrow look really suits you."
3) Steal all his socks.
4) Talk like Professor Trelawney (as in talk in a mystical, quiet, spooky voice whenever you speak to him).
4) When he comes into the Great Hall, point at him and scream, "Hippie!"
5) Or, instead, you can say enthusiastically, "Yeah! The '60s will never die! Peace out, Professor!"
6) Call him "Merlin"
7) Style his hair into spikes
8) Trim his beard really badly
9) Give him a box full of the worst flavours of Bertie Bott's Beans
10) Blatantly give socks to Professor McGonagall and the rest of the staff and hand him a stack of books.