I agree with Istanbul and Venice but I think there are so many better choices than Barcelona (which can be done in one or two day stops on multiple cruises) or New Orleans (which is an easy land trip for North Americans).
Top of my list would be Athens. There are so many sights to see in Athens and neighboring areas that I always plan on pre-cruise trip. Of course, now, after three trips in three years there is less to see.
Next on my list would be Hong Kong. You can spend days at a time in Hong Kong and never tire of it.
Bangkok definitely needs a couple of days to see and is a great pre or post-cruise location. Throw in a side trip to Chang Mai or Seem Reap and you have a great adventure. Next year's trip to Bangkok I am finally going to get to play with the Tigers; have not been able to fit it in on past trips.
Lastly, I think Sydney definitely deserves a few days pre or post-cruise. Yes, you can probably see the highlights in a couple of days but you need to take time to see the city from the water day and night and climb the bridge.
There are many other cities where I have spent time pre-cruise and were wonderful (Buenos Aires being the most recent), but these four, along with Venice would be my top 5.