L.,
You are speakin' my language. Where are you going to school? I am studying psychology and have a vested interest in social psychology!
First, are you actually doing the experiment or do you just have to design it? That makes a big difference as to what you do. How long do you have to do it?
Does it have to be original? Or can you replicate what someone else has already done?
There was a study done (I can't remember who did it or when) but they had participants come in to do a survey, at the end of the survey, when they were leaving, they had a confederate drop some papers and they kept track of who helped in picking them up.
My favorite social psych experiments are the ones looking at stereotypes, but that's just me ;)
Feel free to email me if you want a brain to pick or someone to bounce ideas off of: ____@____.com