Planning well will be the key to the success of your trip at the ports. There are certainly cruiseline excursions available and many of them. You should be able to view them from the cruiseline website with your booking #. They will always be your safest option. They are screended vendors, the ship staff knows where you are in case of delays or problems and they're easy to research and choose from.
There are outside excursion choices as well and sometimes at lower cost. You will NOT want to wait until arriving at the pier and hope to find something. If you're considering outside options, remember the language barrier that will be an obstacle, you will want to use vendors with good reputations, that takes digging and recommendations from experienced travelers. There are many on this site that will, I'm sure, give you their experiences and advise on ones they've used and enjoyed. Third option is, but I wouldn't advise doing this for first time Europe vacations, arranging your own private transportation and doing your own thing. That way you can tailor your tours to exactly the thing you want to see and do. A great option for those who know the area, speak the language or are experienced enough to plan well, keep an eye on times and plan for alternate options in the event of issues, delays, etc. Again, not a good idea at all for a first time to Europe and no knowledge of the ports and geography, travel time needed, amount of time needed to truly see the sites you're visiting, lines at popular attractions, etc.
You can use the Find tab on here and put in key words like the ports you're visiting to gather more info from previous post here as well as reading the replies you get. I hope you have a wonderful vacation but start your planning now if you intend to do other than cruiseline excursion options available.