For Harry Potter, book shops sell tiny boxes with teeny stickers and a 'something' in them - eg one has a snitch, another has the horcrux locket etc. They are quite cute and about a fiver each. Other ideas might be a charm bracelet with a dog (Sirius), a wolf (Lupin) etc. You could also put in a piece of black velvet for a cloak maybe, and book tickets for the studio tours as the 'big' thing to tie it all together.
Spy kit - those glasses/nose kits for a disguise?
Somebody on here once did a Famous Five style solving mysteries box, with ginger beer, string, old fashioned toffees, a torch etc. I was very jealous of that one.
Baking kits, as below, are good and easy to adapt for different ages.
We did dd2 a disguise kit once, complete with non-prescription 'proper' NHS glasses, ears, moustaches, noses, face paint for drawing beards etc etc. She loved it.