If you're interested in having a Harry Potter themed party; perhaps for when The Half Blood Prince movie comes out in November or for any reason (although, who needs a reason?!) here's what we did! While we had our party at our Taekwondo school (and we had more space for activities) I'm sure these ideas would work at home too. Plus, we wouldn't invite thirty kids if we didn't have the room!
I set up four tables and put cheap plastic tablecloths on them in the corresponding house colors. Gryffindor- Burgundy with gold streamers laid on top; Hufflepuff- yellow with black streamers; Ravenclaw- Blue tablecloth with white streamers (couldn't find bronze) and Slytherin, green tablecloth with silver streamers. I printed the house shield on cardstock for each table and taped them to a small orange cone so it'd stand up.
Entertainment- while we waited for everyone to arrive, we sampled the gross flavored "Bertie Bott's Every Flavor Beans" jelly beans. (found the boxes at Borders for 50% off). Gross flavors like vomit. earthworm. soap. booger. rotten egg. Of course, it was a hit watching the brave kids suffer through eating them. We played warewolf tag (get tagged by the warewolf and you are it too) and broomstick hot potato (when the HP music stopped, the person holding the broom is out.) it was hard to hear the music from the squeals of the kids, but that means they had fun, right?
We also had a potions table where the kids could make their own potions to drink. I bought four italian soda flavorings and covered up the labels with my own- Mandrake Juice, Felix Felicis, Skele-Grow and something else (I can't remember now). Make sure you buy the commercial style plastic cups with lids and straws, so they don't spill! The best part- found Pop Rocks candy in tubes, the kids added it to their potion and not only would it sizzle, but you'd get a few candies in a sip and it'd fizz in your mouth!
Made a Hogwarts shaped cake (chocolate with ganache- YUM!) but it was for the adults only. Kids get too messy with that kind of chocolate cake. So, they got to eat frosted sugar cookies with simple piped-on designs (a snitch, lightening bolt scar, and HP glasses).
Let my daughter talk me into drawing a scar onto her forehead with a washable brown marker; and after debating and finally saying "yes" and drawing it on, we realized her bangs covered most of it so it didn't make a difference anyways. :)
Finally, didn't want to spend a ton on goodie bags, so it was just a small bag of jelly beans (I mixed in good flavors with the gross ones) with labels I made saying "Bertie Bott's Every Flavor Beans"; and a Dark Mark tattoo.