This is a good list of pointers to remember when decorating a classroom. I like visuals and would probably be guilty of putting up too much at one time. I loved the question "Do the walls need Ritalin?" as a reminder to tone it down!
This is the only icebreaker that I use with my high school science students! I've used it for over 5 years and it is always my favorite day of the year. This icebreaker activity allows you to 1) talk science on day one 2) randomly group students for your seating chart 3) spend time getting to know your students and 4) allowing them to get to know you!