Barbosa, each port on an Alaskan cruise has something different to offer. Of course they will all have flight tours and fishing tours, however each port is known for something special or different.
Ketchikan is a major rainforest and you are likely to get rain there. The totem parks in Ketchikan are interesting and kayaking is also big here. Fishing is probably its biggest draw.
Juneau has rafting and places to pan for gold. It is also one of the best spots for whale watching. You can also get to Mendenhall glacier cheaply and easily here. This will also be the biggest port a ship will stop at in Alaska. Flight tours onto the glaciers are great here also.
Skagway is extremely small but you spend long days here because site seeing trips into the Yukon are done from here. The Whitepass Railway has spectacular views. You can also take some nice nature tours to Haines from here. It is only a short boat ride from Skagway to Haines.
These are the most common ports to visit on a cruise. Some do hit Sitka and a few new ports have been added on some lines such as Royal Caribbean but the must stopped at ports are the ones I have mentioned.
Spending some time on land is well worth doing. We have done that ourselves. When we did it we did it on our own and not through the cruise line and truly enjoyed spending some time seeing things a little farther inland. So if you can afford it and have the time go ahead and spend a few extra days in Alaska. You won't regret it.