On the last day (or better, night) in St. Petes, at around 1.30 AM, we went to see the bridges. Something that anybody coming to St. Petes should see at least once.
However, when you do it - stay on your side of the islands, so you can be a tourist and go "the bridges are open? woooooow", and not a person who lives there, who usually goes "the bridges are open? oh fuuu--"
One thing to remember is, the bridges open only in summer, so the best thing to do is come around early June for the White Nights.