Sticking with the DIY theme, choose a topic that your child loves and find a kit to match. For example, if they love birds, find a paper pop-out bird kit that kids can pull apart and put together to make their own bird decorations. If they like the beach, find a DIY ocean animal kit - you know the ones made from wood that the kids can paint themselves. A mini jigsaw, materials to make a sock puppet, or a book will do the trick too.
Personally, if I was suddenly responsible for the goody bags at a birthday party, I would grab all the jars from my jar cupboard or recycling bin, throw on a large batch of cookies, find repurposed ribbons in my craft cupboard, and get to work. The cookies are stacked in the jars, the ribbon is tied around the top of the jar and boom… done.
If none of these options stand out to you, think of consumables instead of junk that sits around. Choose a bag that is reusable or at least made from home compostable paper and stick to the affordable option. There’s no point in spending money on a party favour that is going to sit on a kitchen bench collecting dust for the rest of the year. No matter what it is, that is not sustainable for anyone involved.