Harry Potter: Dragon's Tooth NecklaceHarry's a wild card. The Boy Who Lived seems the type to pick out the most random, yet amazingly awesome gift anyone has ever received, like a dragon's tooth necklace. Whatever he chose, it would probably be purchased at the last minute, expensive and hastily wrapped - but it's a dragon's tooth so who cares?
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Ron Weasley: A Chocolate Frog, or a Weasley SweaterAnyone lucky enough to count Ronald Weasley among their friends could expect to get the kind of gift that may not be flashy, but that you can tell a lot of effort went into choosing and saving up for. Knowing Ron, it'd be edible too. A chocolate frog perhaps? If Rowling's lucky, he'd even ask Mrs. Weasley to make her her very own Weasley sweater.
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Hermione Granger: Handbound BooksThe queen of practicality with a heart of gold, Hermione is the most likely to give Rowling something like a super impressive set of handbound books or monogrammed fountain pens. Or, she'd donate to a charity in Rowling's name. It might not be the most exciting gift, but Rowling wouldn't be able to deny that her heart was in the right place.
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Luna Lovegood: A Super Meta BoxAh, Luna. Sweet, thoughtful, weird Luna. She's the type to hand Rowling an extravagantly wrapped box with a giant bow on it, and when she opens it to find that the box is empty, Luna would look at her with a dreamy smile and say the gift is invisible, obviously.
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Neville Longbottom: A Gift Card to Honeyduke'sJust be thankful he managed not to forget.
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The Weasley Twins: An office/ballpitCome on - how badly do we all wish we were friends with the Weasley twins during school? If Rowling was, she'd probably wake up to her room being turned into a ballpit, with things only getting better (and crazier) from there. (Just in case you haven't noticed, we're completely ignoring the death-that-must-not-be-spoken-of.)
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Draco Malfoy: Probs NothingMalfoy doesn't give gifts, he receives them. He'd show up at Rowling's party asking what she brought for him. If he was really feeling generous, she'd be lucky to get a cheap, re-gifted inkpot.
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Hagrid: A monster from the dark forest, duhExpect something dangerous, illegal, and with lots of teeth (and no, it's not a puppy, unless that puppy has three heads or can breathe fire or something). Don't accept anything dragon egg-shaped, and for everyone's sake, don't let him make the cake.
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Snape: Potion bookPotions are useful, and everyone should learn as much as they can about using them to ensnare the senses and enrich the mind and all that. (Snape is secretly a total fanboy when it comes to potions and his friends would know that - if he had any.)
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Albus Dumbledore: An ancient magical artifact + a rad adventureIf Dumbledore were a guest at Rowling's birthday party, she could expect to receive a super mysterious magical artifact, delivered with a very cryptic message. It would take days, but she'd eventually realize that it's the most thoughtful present she's ever received in her entire life.
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Vodelmort: Hateful curses (aka don't invite this dude)Two words: avada kedavra. He might open with with the cruciatus curse, though, if he's feeling festive. A word of advice - don't invite the Dark Lord to your birthday party.
