Step1
Invitations: Little rolls of parchment. I found this browny-beige paper that looked like parchment. I typed up the info and printed it on the paper. Roll it up, and tie with a ribbon. Easy, and cool!
The front door: On one piece of poster board, write Platform 9, on the next write Platform 10 and on the last, write Platform 9 3/4. Stick the 9 3/4 one on the door, and the others on each side.
Step2
Activity Number One: Quidditch
This activity is great in the summer time, and can be played in the backyard. Divide the kids into two teams (best wtih 7 on each team) and assign each person a position (3 chasers, 2 beaters, 1 keeper and 1 seeker). You can draw these out of a hat to keep it "fair" if necessary. Tie a piece of red cloth to all the members of one team (gryffindor) and green cloth to the other (slytherin).
The balls: One red dodgeball for the Quaffle, 2 black for the Bludgers and one small one (you can paint it gold if you wish) for the Snitch.
Setup: Hide the Snitch somewhere in the backyard beforehand. Create the goal hoops. I was able to rig up something cool with some hula hoops, but baskets or bins work fin too.
The Game: Chasers will pass the ball between eachother to try get it in the hoops. Beaters will throw the black ball at others. If someone is hit by a black ball, they have to stop, and stand still for 5 seconds. The seekers will look around the backyard for the snitch. When it is found, the game is over!!
Step3
Activity Two: Potions Class
Making beverages. You can choose an easy recipe, and prepare ingredients beforehand. Smoothies or lemonade work well. Kids can work in pairs, and take turns using the blender.
Step4
Activity 3: Shopping at Diagon Alley
This is where the kids put together their own loot bags. Have a variety of Harry Potter or wizarding merchandise layed out on tables. (I got Bertie Botts Every Flavour Beans, Chocolate Frogs, Blood Pops, harry potter glasses, wands, trading cards, and little magic kits and such).
Have children walk around the table and pick up one of each item and put in a bag. I also found these mini-cauldrons that the kids used for their bags.
Step5
Activity 4: Eating at the Three Broomsticks
This is pretty much just the kids eating pizza, cake etc. I got a nice harry potter cake, but i didnt want to play around with the food too much. Its easier going with party classics like pizza, chips etc. You can give the food cool names though if you wish :)
Step6
Activity 5: The Yule Ball
With any extra time, you can play some music and have the kids dance around. I doubt you will have more time, and this is just an activity to keep them occupied as their parents pick them up.
Novice 
