Our Advent calendar 2010 were 24 pretty little houses, filled with tiny chocolate, gummi bears and crayons.
They were made from those cardboard roles in toilet papers, kitchen paper, gift wrapping and so on. Cut into different sizes.
They were painted using acrylic color (one or two layers). It looks best to mix in a bit of contrasting colors instead of having even colored houses walls (which still look nice, though). Windows and doors were only roughly painted. You could also use stronger papers or thin cardboards, any pens and colors for building those houses. And a bit older kids might enjoy to help you or even making there one houses.
I added some details such as numbers and clued some cardboard at the bottoms to close them. The roofs are made using leftover wallpapers/wrappings/scrapbooking papers. Cut them into kind of circles: Low roofs will need about half a circle, spiky roofs will need less. I just stapled them as I only got tiny moments for finishing this project.