I found this gorgeous Hawaiian lumunaries project at DIY network. There are so many fun designs you could come up with to decoupage onto vases and what an impressive display they would make.
For a more kid friendly decoupage project, try decoupage with paper napkins. They did a flower on a flower pot at CraftIdeas.info but many other containers would work.
This decoupage Halloween candle project by Jane Lake is my favorite and you can see it on All Free Crafts website.