I LOVE party planning wheredoistartmrs! My problem is that I churn out ideas and then run out of time/money to execute them!!!
PLEASE tell me when to stop! this is the best distraction ever!
Make the invitations. Send out letters from Hogwards. Print them in a lovely italics script, "age" the paper with cold tea then fold them and seal with a blob of red candle wax. That's also a task your DD can be heavily involved in! Use the text from Harry's letter in the philosopher's stone as your guide and tailor it to your DD!
Herbology
Identify the edible! Offer various greenery, I'm thinking everything from basil and mint still on the plants so it needs to be picked" To cucumber, lettuce etc and encourage a "scientific style" tasting of professor sprout's plants! (You can then leave this out for the food later as well!)
Are you a cake pops person? (I'm really not!) but I'm sure you could decorate cake pops to be golden snitches, quaffles, bludgers!... As I'm highly unlikely to EVER attempt a cake pop I would probably make these on either round biscuits or crackers
Liquorice wands (normal liquorice with a chocolate "handle")
Cake wise. DON'T stress yourself out. There's nothing worse than running the party, creating and preparing loads for it and then becoming totally over worked up over the cake, especially if you're not that confident. Yes you can buy one. OF COURSE YOU CAN! just get a little creative (or rather, get the girls to!) You can buy ready iced, plain sponge cakes. Just a blank canvas. (something like £6 in tesco) buy some writing icing and set your house elves to work decorating a "Magical" cake for your DD. provide sweets and other gubbins, ensure that they stick to the whole "Magical" idea and EH VOILLA! A totally unique, made with love, PERFECT cake created by house elves!!!