The national park 'gold rush' museum is worth a visit, and free. Easy walk of dock. Lots of very good amber, good prices too. The railroad trip to the gold rush fields is well worth it. Tip: the new library has free WiFi, but doesn't until 1pm on weekends. If you sit by the front door you can pick up a moderate signal, however.
This was 4th Alaska cruise, so no excursion (we did $175 thermal spa package instead [well worth it]). However, recommend touring the 'red light' historical district [next to dock] then walk up the road that follows the river and go to the totem museum. Very good museum.