Take the bus from the dock to Mendenhall Glacier. It's about $12 round trip each. Small extra amount to enter the visitor center.
A whale watch excursion is always great IMHO.
If the weather's clear and you're not afraid of heights take the Mt Roberts tram up the hill. Don't bother however if it's overcast.
Walk around town. It's a bit hilly but there's a nice museum or 2 and the Russian church, etc. Plus shopping of course......always shopping.