Mage Robes
I just read in a book (Storm Front by Jim Butcher) that wizards wear robes because their laboratories are so cold. I like that idea. I'll take it step further in an adventure to say that when the wizard leaves the house, he puts on normal clothes. The other adventurers waiting for him can look on, confused. "What? Did you think I was wearing that out of the house? It's sweltering outside!"