One great program in Ubuntu is called Sweet Home 3D and you can find it right in the Ubuntu Software Center. Sweet Home 3D is an interior design application for picking and placing furniture and design elements onto a 2D plan, and visualize your idea into 3D. Sweet Home 3D comes in 16 languages and is actually a java app.
This really is a lot of fun to play with and great if you want to see what your rooms might look like without actually having to move furniture around.
Once you have your designs the way you like them, you can export to a file such as PDF, bitmap or vector images, common video formats and even 3D render files. This program really is easy to use and you'll have fun and learn it hands on.
So what are you doing this weekend? Moving furniture around or designing your room with Sweet Home 3D!