They're a bit like marmite
I've been on a few and they are a great way of seeing different parts of the world, albeit it in snippets.
Research, research, research the ports you stop at and decide what you want to do/see. Do your own trips, the ones the ships provide are over priced.
If you are on FB join a group for your cruiseline and or ship.
Search FB for the cruise code/name and date you are doing and there's a good chance there'll be a group set up for it. Join that as there will be lots of info on whats what, you can even get with fellow cruisers to share diy trips.
Enjoy it