From places like K-mart you can usually buy a bag of dinosoars or farm animals for up to $4...I sometimes put a couple of those in each goodie bag and the kids love them. You can also get sticker books with themes like fairies, farm animals, dinosoars etc for around $2 from your cheapo shops - I buy a couple of books and then rip out a page each(which has around 10 stickers on it) and put that in the goodie bag.

Oh and the cake sounds really good too.