Choices, choices.... I would want
1) A box of Weasley Wizard Wheazes merchandice
2) A bottle of liquid luck, Felix Felicis
3) A copy of Hogwarts: A History
4) A moving picture
5) Chocolate frog cards, of all my favorite wizards
I suppose (besides the obvious choice of a wand, with which you can do basically anything) maybe Harry's Invisibility Cloak. I also wouldn't mind the Time-Turner. Or an endless supply of Felix Felicis. Oh! And, of course, a Firebolt. Flying on a broomstick? Yes please