I grew up near Trondheim and can recommend a few sights to see:
1. The Nidarosdomen Cathedral - a tousand year old cathedral with lots of history around.
2. For a walk: The Bakklandet area, across the river from down town. Old buildings with small shops, cafes etc.
3. The Tyholt tower - a rotating tower with a great view over the city and surrounding areas.
4. Ravnkloa Fish market - a fish market - not as large as the one in Bergen, but still a nice atmosphere.
5. Stiftsgården - the Royal Residence in Trondheim
6. Ringve Museum - a museum covering the music history
The ship is probably offering a city tour which covers most sights if you don't want to concentrate on one or two on your own
If you went to Oslo, I could come and take you on a private sightseeing myself