Typical witch and wizard stuff would be appropriate and easy.
Hang bats, owls, spider webs, witch cut outs and wizard hats from the ceiling.
Place signs here and there with arrows pointing to "Hogwarts" (at your main entrance), Gryffindor, Hufflepuff, Ravenclaw and Slytherin, and Diagon Alley.
Place brooms around the room.
Hang some twinkling Christmas lights, place some glow-in-the-dark stars, moons and of course, lightning bolts about the room.
Sprinkle gold stars on the refreshment table.
An appropriately-placed ghost (decorate a white helium-filled balloon with some white or metallic streamers) representing your favorite from the books (Nearly-Headless Nick, for example) might add atmosphere to the bathroom or entryway.
Around Halloween you might find inspiration in your local craft store, for example leaky cauldrons and bat decorations.
Christmas time gives us silver and gold star garlands...if you do enough advanced planning for your party you might remember to pick up a few strings! Use gold paper plates and cups, as well as metallic-looking forks and spoons.
For a custom look, glue gold stars, moons or other Harry Potter motifs on to each gold plate and top with another clear plastic plate. A drop or two of glue between the plates will keep the units unified. Do the same with gold cups, decorated individually and inserted into clear plastic cups.
Finally, have some music playing in the background. Choose a spacey New-Age assortment for just the right atmosphere.
If your owl is otherwise occupied, have a large hairy man on a huge motorcycle deliver manila- colored envelopes to invitees, addressed to:
The Cupboard Under the Stairs
City, State, Zip
Use green ink for the text, and seal with purple sealing wax imprinted with the letter 'H'.
As with all party invitations, remember to include the following information:Will the guest be assigned a character name before the party?
Are costumes required or suggested?
Should the guest bring a smock or wear old clothes?
Does the child have any allergies you might not know of?
Also, include a pick-up time or prepare to suffer the consequences!
If you are REALLY carnivorous, and want a truly authentic Hogwarts Start-of-Term Banquet in the Great Hall, then by all means serve a sit-down meal of:Roast Beef
Boiled, Roasted and Fried Potatoes
Peppermint Humbugs (whatever they are!)
However, if you want something a little more modest, serve:Bernie Bott's Every Flavor Beans (multi-flavored jelly-bellies would suffice)
Chocolate Frogs (purchase frog-shaped molds and chocolate melts at craft or cake decorating stores).
Poor Harry was a cake-deprived child, so in his honor make a nice gooey chocolate cake, decorated in wizardly-fashion with broomsticks and Quiddich balls, or small plastic owls, cats or toads. Fringe picks (for cocktail garnishes or small hors d'oeuvres) make a smashing magic wand to stick in the cake or sandwiches.
Magic Potions are the obvious choice for beverages: Use variously colored canned fruit drinks, from Gatorade to strawberry punch, displayed in clear glass or crystal jars or pitchers. Let the kids measure out and pour portions of each drink into a punch bowl and have the guest of honor add a clear carbonated soft drink such as 7-Up or Ginger Ale.
Or try this:
Prepare concentrated grape, strawberry or raspberry juice half-strength. Pour into star-shaped ice cube trays. Serve clear soft drinks to the children, and have them add the colored ice cubes. Abracadabra, the drink changes color as the ice cubes melt!
Make and decorate wizard hats. A seamstress can probably whip off a few capes or invisibility cloaks for the kids to decorate with fabric paint or fabric glow-pens. Or have all kids decorate a single cotton robe as a souvenir for the guest of honor.
Have the children paint lightning bolts on each other's foreheads. Give special recognition to the most interesting creations.
Use ribbons, confetti and 3-D paint pens to decorate broomsticks for the Quidditch team. Or make miniature versions out of short dowel rods for the kids to hang in their rooms.
Design t-shirts with a quidditch team name and logo.
Make Harry Potter "cootie catcher" fortune tellers.
Have the kids provide photos of themselves in advance. Scan them into a versatile graphics program and create trading cards with "famous witches and wizards" complete with a short bio. Print out enough for each child to have a whole set and laminate them.
Make a mosaic out of jelly beans.
Using cardboard boxes, make a train to Hogwarts. Have each child make a decorative banner representing his or her residence house at Hogwarts.
Make a magic wand, either out of wood or plastic tubing.
Games and Activities
Make wizardly cookies and have the children decorate them. This makes a good opening activity.
In your yard, set up a number of stations with small tasks at each. The first could be cupboard under the stairs, proceeding to various stores at Diagon Alley, on to King's Cross platform 9 3/4, and the last Gryffindor. Kids could take the 'train', made of cardboard boxes, from station to station and be required to mix and drink the magic potion, ride the Nimbus 2000 through obstacles, find hidden keys, answer a simple Harry Potter trivia question, unscramble a related word, and dig for a hidden treasure in a bucket of sand, for example.
Play a simple game of Quidditch, perhaps by sweeping balls into a goal, tagging opponents with foam balls, racing, dodging and tagging. Teams can be selected from a sorting hat, which if you are really ambitious can have recorded message or a walkie-talkie receiver inside, singing the sorting hat's song and deciding each child's house assignment. T-shirts or pins can designate team names.
Slay the dragon by whacking a dragon pinata.
Play pin the pig tail on Dudley.
Guess the number of Every Flavor Beans in a jar.
Play Harry Potter Trivia.
Any number of hiding/finding games can be adapted to the theme.
Make W-I-Z-A-R-D bingo boards.