1. We run a funny t-shirt contest and we sent out a national press release, From it I got a few articles and a half dozen ten minute radio interviews. We got thousands of entries from the effort.
2. I have heard (from somewhere recently) that the best advertising going is to combine billboards with local ads run during drive time.
It all depends on what the goal of your campaign is. Billboard companies like to tell you that millions of comuters see your billboard everyday. Also, radio's reach is rather limited to a narrow spectrum of listeners at the moment your spot gets played. Most media buyers pick two radio stations with similar demographics in order to increase the likelihood that the target audience hears the spot.
Stunts are always a great way to get PR, but again, what's the goal of the campaign?
We've actually had a lot of success promoting a trade show with bus billboards. I'm fond of the panel on the rear of the bus because the people stuck behind the bus cannot help but have your message seared into their brain.