Just about everything is allowable :) In short, if we don't know you were there at the end of the night, it is allowed.
Not allowable
Anything that causes damage to the physical venue, such as nails, screws, etc. This would include tape or adhesives on painted surfaces. This ends up damaging the paint.
Some helpful hints
For hanging on wood, many have found the command strips to work well.
Although allowable, we do discourage feather boas as they make a huge mess.