Basically, as long as I can paint back over it at the end of the year, I'm pretty free. They prefer we use sticky-tack or removable double-sided tape though...(aka no nails). There is a tack strip to hang things off of around the top perimeter of all the rooms too. Let's see what else...general guidelines really--nothing that will cause a massive fire, nothing that will cause permanent damage to the walls or floor.
In short, anything goes, as long as I am able to change it back at the end of the year.