Hagrid is the much beloved half giant in the Harry Potter series. We first meet him bringing the newly orphaned Harry to the Dursley's home. Dressing up as Hagrid may be a large task. He is "almost twice as tall as a normal man and at least five times as wide," a quote from "The Sorcerer's Stone."
Grow a bushy black beard and long wild hair. This is one of Hagrid's identifying features. It is wild and unkempt. You can buy a wig and beard at many costume shops or online at Alivan's.
Dress in groundskeepers cloths. Hagrid is in charge of the keys and grounds at Hogwarts. He wears outside work clothes and boots. Wear large clothes and add stuffing. Hagrid is a large half giant.
Wear a moleskin coat with many pockets. Hagrid's coat is known to hold contain many things in the large number of pockets. He has on occasion pulled out a teapot and several chipped mugs. When he first meets Harry in the "Sorcerer's Stone," he even pulled Harry's birthday cake from his coat.
Carry a pink umbrella. When Hagrid was expelled from Hogwarts his third year his wand, a sixteen inch oak wand that was rather bendy, was broken. His wand was embedded in a pink umbrella. He is forbidden to do magic, however he sometimes does with this umbrella.
Find a magical creature to carry with you. Keep in mind that Hagrid likes the more dangerous of magical creatures. He has a boar hound named fang. He also has an acromantula, a hippogriff, a dragon and a hellhound. You can find replicas of these creatures at toy stores, costume shops and online.
A nice addition to any Hagrid costume--a magical motorcycle. He is seen on this several times. He brings Harry to the Dursley's on it in the Sorcerer's Stone. Hagrid also takes Harry away from the Dursley's on his motorcycle in the Deathly Hallows.