Go to this very useful web site: boards.cruisecritic.com - scroll down to the Ports of Call section and click on the link for Europe. You will find loads of info of how to DIY!
In addition on this site you can click on the link for your cruise line and find loads of Q&As on it - and if you join - you can click on the link for Roll Call - find your ship and sail date and then 'chat' beforehand with fellow travellers and perhaps even link up with them for some independent travel. This is what I have done on my last 6 cruises and it has worked out very well [and much cheaper than the cruise lines tours!] Happy sailing!