I love finding ways to use items that i have lying around or things that would normally get thrown away: things like cardboard, old pencils, old plastic parts and models, etc. As a kid, I also collected Lego and had a lot just sitting in storage. I decided I would build my structures use these old blocks. I tell you, it was so fun pulling these out again!
I pretty much took over the living room, but my wonderful wife didn't mind!
The first building I built was the freight depot. Its the only building in a color that is normal looking, but hey, its fun... and free!