Depends on what you are interested in
There is the Fort, El Popa (monestary) with a great view, the old town, and of course the emerald shopping.
I would do a ship's tour, there was some shopping at the dock but not a lot there
If you like coffee buy some beans/ground coffee to take home ..go past the Juan Valdez stand to the shops for better prices or if you go to town shop there.
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012