This is my HP party (for 9 yrs old) - it took us 3 hours.  I kind of changed the story, especially for placement reasons (in the 4 houses). So, we had a Unicorn (Hufflepuff - earth? - skillful), a Dragon (Gryffindor - fire? - did courageous things), a Merpeople (Slytherin - water? - potion masters) and a Hippogriff (Ravenclaw - air? - found clues)  teams. The invitations were written on a scroll paper - head girl (my girl) celebration at Hogwts. I folded the paper like an envelope and sticked an eye (from Lewis Craft) to close it. The story goes like this (we, adults, kind of started making a fuss when all kids were there). HP has been kidnapped by V. and transformed into a baby (we had a 1 yr old at the party). We had to find a thing (the Phil Stone), which was needed to save HP (or he will grow younger and younger...). But IT was hidden and all we had was some riddle (you can use invisible ink for this one - write with lemon juice, put paper on smth hot). The riddle said that in order to find the stone we had to go through a maze, in teams of four (a dragon, a mermaid, an uni and a hippogriff -- each kid had to do a specific thing during the quest according with its 'skill', see above), and in order to find the maze, we had to make the 'silent potion'. Also mentioned that the IT is something that changes copper and iron into gold and gives you everlasting life (the Phil stone), and that the stone is guarded by the Azkaban guards (dementors). But, in order to escape the dementors, the kids needed some 'rings of power', which can only be obtained by raising a 'mangafella' (an animal - plastic piggy - that if put in a dragon egg shell will grow and become in turn an uni, then a hippogriff, then a dragon and finally a mermaid which will make the ring and leave. If you were not careful though, it might remain a dragon and eat you).   So, we began. The teams were of course selected by the hat (I took a halloween hat, sticked paper eyes and a mouth on it, and put some strings to  make it a marionete). The hat sat on the speakers, and talked (moved at least). Unfortunatelly, nobody saw that the hat 'drank' some unicorn blood and as a result was kind of 'funny' -- kept saying smth like 'Griffindors, everyb wants to be a Griffindor' and placing all kids in the Griff house... (by mistake). The unicorn blood was a sparkly green moisturizer (hat had some on his mouth). When chosen, everyb was given a robe (made out of  black garbage bags), a wand and a purse (which was in fact the loot bag - they had to 'gather' their -labeled- presents). We also took a digital picture of the child (and played aplauses and cheering and thrown confetti) with its team's coat of arms (we coloured a lot to those). I've taken before some picture of me dressed as a 'silly' dementor (with flowers, balloons etc.) and during the party my hsb. printed an 'Order of the Phoenix member certificate' for every child, with his/her picture and the 'silly' dementor somewhere in there too. The Silent Potion. Everything has to be done in total silence. We had 3 teams (12 kids). Each team had a labeled jar (with some number). Every team poured in the jar oil (2 inches), then water + colour (diff colours). When said so, in total silence, each team in sequence will pour in its 'dragon tooth dust' (flour, salt and, the last team had some efervescent powder - alkaseltzer works wonders). When the last was put in (the efervescent one) - we started a very very loud scream tape recorder (actually we had four screams, the last one being a very funny - man - scream -- to ease the tension) -- the winning potion.    We had three rooms (each with a number on the door) and we started with the winning number. There they used clues (go to the comb - there find one puzzle piece - and a present-, go to the blue cupboard etc.) to find a map. In fact I've printed the map on one side of the paper, and on the other side I've printed an HP picture. Then I have divided (perfectly equal - since we had no time) the paper in 3 (for every team) and next each part I've divided in 6. Every team had to find their 6 clues (the pieces of paper) in random order and then recreate their part of the puzzle, then we put everything together - the map. In the next room we had a 'talking bridge'. I made a bridge (from coffee tables) and placed some invisible strings on it. I've took some 'howlers' from the internet (do a search for 'send a howler') and played them (on the pc) everytime the kids touched the strings. Then they had to 'find' all kind of things - the map told them where.   Last room had a lot of baloons in it. I've put in almost all the balloons something (water, confetti, gold powder, and, of course, presents) and they had to blow them up with a plastic fork. In the balloons were also 12 (Easter) egg shells (coloured as dragon eggs).   We also found the mangafellas (which I put one day before in very hard gelatine and tumbled them in sodium bicarbonate) and washed them (in water with lemon juice - made some foam). Then I gave them some silly putty I've made before (corn starch, water and green + yellow food colour - DO NOT PUT IN THE SINK!!!) to put the mangafella in a cube (which of course they couldn't) and then in the egg shell. Next (while somebody took the egg shells and replaced the mangafellas with the rings - hid them in the basement afterwards) we practised the 'Expecto Patronum' spell for dementors. We entered the basement and went through some elastic bands, like in spy movies -- they were 'dementor traps' ('they' put some - Christmas - bells on the elastic bands to hear us come in) -- btw, the elastic bands were mandatory in order to protect the dementor from the kids.  Finally, we found the rings. Immediately the dementor (my hsb.) came (we also had some scary Halloween sound tracks), we spellbounded him and he ran. Next we performed (in front of a door - inside we were supposed to find the dementor) a transfiguration spell ('Transfigurare'), and when we opened the door we found a 'Sponge Bob' pinata, dresed as a dementor, which started 'laughing' (like Sponge Bob), and then singing its team song. Somewhere in the room also was the Phil Stone, hidden in a pineaple (where Sponge Bob lives).  Through the whole house were lots of 'tricks' and 'traps' -- in the bathroom a soap that makes your hands dirty, all the doors they had to open had a trap (like a balloon blowing - there was a string on the door that will thrust the balloon in a needle, when the door opened) etc.