I've spent a lot of time on lesson plans over the past week - about 2 hours a day (except weekends and the Philadelphia Folk Festival). I've also spent a lot of time in my own classroom, organizing my supplies and decorating my room with things I think are important. I would distill them into three major categories: practical decorations; interesting decoration; and funny decorations.
Practical decorations: in any math classroom I think it is important to have information that could help kids who get into a jam. After all, in real life we do similar things with the Internet, right? So I have posted information on place value, fraction conversion to decimals, and common square roots.
Interesting decorations: the famous quote from Martin Niemoller; odd geometric views of the Earth (imagine our planet shaped like an hyperbola); a history of Benjamin Banneker. These are mainly for the oddly intellectual kids who want to read or think about something at the same time as math class.
Funny decorations: I think it's important to not take yourself too seriously so I have t-shirts of various design with funny math-related jokes on them. Additionally, I delved through the numerous demotivational posters to find one that wasn't too mean.
In a more practical matter, I am still seeking to provide my students with Chromebooks for use during the year. All I need is a grant of $5000. I have not had much luck with local bank branches yet. Someone suggested if I were to post my need online I would get some donations. If $40 provided a kid with a laptop in my classroom for the whole year, would you donate?