Theme up party games, so instead of "pin the tail on the donkey" it could be the scar on Harry or the flames on the dragon or the wand in Hagrid's hand....type of thing.
Instead of party bags, similar to the lucky dip idea, wrap the small things into a large parcel - for pass the parcel - with something small in most layers (maybe a multi-bag of haribo - the mini bags - for most layers with a couple of Lego HP figures and then something slightly larger in the middle) - so most guests get something. And other prizes in other games to make sure each guest does get something.
Make a Pinata in the shape of an owl or large spider to bash in the garden (carefully!) and put individually wrapped sweets in that. Just use a balloon and papier mache for the main body, add cardboard wings or spindly legs for decoration.
A craft activity as people are arriving is good. Making wands. Or later on you could let them decorate buns - a couple of different coloured icings in bowls, some strawberry laces and liquorice cut into bits, smarties, etc. And you could hand out a pinch of edible glitter or edible sparkly stuff if they want to make them "extra magic" (don't put that on the table or it will all disappear in seconds!). Let them either bring the buns home or have them as dessert.
Do you have any star or moon shaped biscuit cutters? You could either make cookies that shape, or even just cut the sandwiches into that shape (ok the sambos means lots of waste and crusts - but you can put those aside for family to nibble later).
"Magic Wand Kebabs" - either vegetables (to BBQ) or fruit (cold or BBQ) on bamboo skewers
Some kind of "Potion" mocktail (basically fruit squash, with some bits of fruit bobbing in it or maybe some raisins as "crushed beetles" or something....) in jugs to drink, and "Elixir of Life" (plain water) for those who don't want that.