For me planning an affordable birthday party means not buying store decorations. By that I mean no theme parties where everything matches. Try decorating with lots of balloons.. taped to the ceiling... no helium needed. try using colored pencils, crayons, paints, and markers as color... How about printing off theme decorations and coloring them? Kids don't really need goodie bags but if u insist just put pencils, bouncy balls, or candy in it.. things you can buy in bulk at a novelty store for super cheap. or make a snack and wrap it up.