On Mumsnet, party bag fillers all go "straight in the bin". My children can't be the only ones who are usually still playing with the toys days, even months later. Sometimes they even spark a new interest.
Things they've kept, or used up, include:
those pens with 4 different colours of ink
those plastic pencils with 10 different colours you can rotate through
rhinestone stickers
a little craft project
small notebook
rolls of stickers
sticky gecko
bubbles
a small dress up mask, or prop
a balloon
small plastic animal
small car
party blower
Add a mini bag of haribo and you are good to go.