Bergen is a lovely town and quite easily seen on foot. We happened to be there on 17 May, which is Norway's National Day, so most shops and museums were closed. The funicular to the top of Mt. Floien is very fun and on a sunny day the view from the top should be spectacular.
No need to purchase any shore excursions in Bergen. The town is an easy 5-10 minute walk from the pier, and many of the more popular sights/museums are within walking distance. With the alloted time in port, there should be plenty to do on your own.