If you are trying to stretch your dollar, pick one port and "do it up" with a great excursion. We did a sea/land trip and picked a couple of places we wanted to spend extra for the excursions. At the other ports, we "did it on our own". Juneau, Icy Strait, and Ketchikan were 3 of our ports also.
Juneau - we actually rented a car for the day, drove out to Mendenhall Glacier, went to the chapel in the woods, got some amazing pictures, drove over to Douglas, etc, etc. We also left ourselves time to walk around town later. Inexpensive way to spend the day and lots to see. You're not likely to get lost....the roads leading out of Juneau all just "end", but the sights outside the city are very pretty until they do.
Icy Strait Point - we were there in 2007. Maybe things have changed...but if they haven't, there really is little to see there. We walked into town and back, but I am using the word "town" loosely. A lot of people did a whale watch out of Icy Strait and said it was great. If I ever went back, I would skip town and do the whale watch. As we were leaving port the whales were everywhere, so I suspect the whale watch was great that day.
Ketchikan - this was where we opted for a pricey excursion and did a float plane to Misty Fjords. We booked privately and the trip included landing inside the park on one of the "ponds", so we could get out and walk around for awhile. The trip was amazing and we saw a lot of wildlife on the mountain tops.
One word of caution - if you do decide to book any type of plane trip - DO YOUR RESEARCH - and be sure your pilot is someone who has flown that area a lot. From what I understand, a lot of companies hire pilots to come up for the summer to fly excursions. These guys might be great pilots, but they don't know Alaska or its weather. Misty Fjords, in particular, can have fog roll in pretty quickly. You'll want a well-experienced pilot, if that happens. The woman we hired was very well-known in the area and has been flying there for years and years.
I think a few others have said this - but I would agree, if you can spend even a few extra days on mainland Alaska after your cruise - do so!!! And try to get up to Denali, if nothing else. If you DO get to Denali, do NOT do the Denali Natural History Tour. DO the Tundra Wilderness Tour. It will take you all day, but you go in much further and will see a lot more wildlife.
Enjoy!!