We just got back from this exact cruise with Princess, loved it. We stayed a few extra nights in Anchorage too at the end.
My advise is don't skimp, yes the shore tours are a bit expensive but you may only be there once so book something to do at each stop. If you plan on just walking around town I don't think your getting the most out of your trip - the 3 towns you stop at are not big you have plenty of time to walk around after one of the tours. We missed out on one tour as it was fully booked before the boat sailed so my advise is book prior to getting on the ship. It all worked out for the best as we did a plane ride around Denali when we got to Anchorage - expensive, yes, but memory and experience that will last my lifetime and probably one I won't get the chance to repeat (so may other great places in the world to visit!).
Don't skimp
Bring trousers with elastic waist
Don't drink the urn coffee in the buffet
ENJOY!