Have you considered the impact of that cheap and often overlooked decorating media- balloons? I have frequently noticed the number of store windows that are decorated with balloons and they look colourful and attractive. (Shop displays are a great source of inspiration for decorating kidmin areas, by the way.)
Check out some of these balloon designs.. Clicking on any of the designs in the gallery will take you to more information and instructions for how to make these colourful displays. And, most do not require a helium tank. Check it out and have fun designing your areas.