Cruise excursions are highly overrated. Yeah, they're convenient, and sometimes you have to book a cruise excursion due to timing in a particular port, but you can do just as good, if not better, booking your own excursion if there's something you really want to see or do. Going on a cruise excursion also means you're going with 50 of your new BFFs, some of whom may be a swift pain in the...you get the picture! A large group moves around only as fast as the slowest person, which not only slows the group down, but your itinerary can change accordingly, and you could end up seeing only a fraction of what you thought you would (and paid for). Been there, done that. There's always someone "special" who can't tell time, gets lost, does their own thing at the expense of others, or complains the whole time, which taints your excursion experience.
Having said that, not all cruise excursions are bad or a waste of money. I've gone on some good ones. But I've also found that sometimes just taking a cab around the city (be sure you negotiate the fee up front, not at the end), or taking a cab to the beach, or finding a tour on your own is more enjoyable. One way to find out what others think about a particular excursion is to check it out on Cruise Critic.
If you want something stress free and don't want to leave the research to someone else, then go on an excursion with the cruise line. But keep in mind, shore excursions purchased through the cruise line will usually cost more than doing it on your own. The cruise line buys excursions from local tour operators and charges you for their time in organizing, managing and getting you to the excursions. They are in the business to make a profit and in order to make a profit, they charge you extra for the same tour you could buy on your own.
The cruises that we're looking are port intensive, which means we are in a different port and sometimes different country, every day. Most of the places we'll visit have towns and attractions close to where the ship docks, so we can explore on our own without paying for expensive cruise excursions. If you want to go back to the ship for lunch, you can. If you're tired, you can go back to the ship and chilax on deck. Or you can go out all day to explore and see as much as you can. In most cases, the ship is docked for about 10-12 hours, which gives you around 8-10 hours in port.
Booking an excursion on your own means you may have a private tour, or tour with a smaller group vs. 50+ people. You can usually see and do more because you aren't waiting on a lot of people, small vehicles can go places where large buses can't, and tour guides are more inclined to personalize a smaller tour, and you can even change the itinerary at the drop of a hat. A smaller tour can be more expensive than an cruise excursion, especially if there are just 2 of you.
Don't be afraid of booking your own tour. You'll get back to the ship in plenty of time. This is how these people make a living, and if they don't get customers back to the ship, their business won't survive. The economy in most of these cruise ports is dependent on tourists.
If you're just going to explore the city in port, be sure to check out the web and learn more about what there is to see and do. Last thing you want is to find out about a great museum or shop or activity afterwards. Check out YouTube videos that others have posted about their travels, which often include some great tips about those ports. Or, you can stock up travel books - Lonely Planet, Frommer's and Rick Steeves are pretty good. Read at home; don't bring 'em with you. Do you really want to haul 20 pounds of books around Europe? Better choice - download to a Kindle or Nook.
I usually check out what the cruise line has to offer, and then surf the web to see if I can find a better deal or better tour, on my own. I've been pretty successful on both European and Caribbean cruises and would rate my success rate at 99%. Loved the riverboat cruise in Rome on the Tiber River, but it was shortly after we arrived after our long haul flight from the USA. A nice relaxing ride down the river and jet lag = lots of sleepy people.
Sometimes just walking around town is just as good, if not better than an excursion. When we were in Rhodes (Greece), we went off on our own in town. We saw the sites, we did some shopping, had a little lunch, and then stumbled on the most amazing rug store. On the flip side, when we docked in Pireaus we had a tour scheduled and headed to Athens (about 9 hours). We were in a van (our own private taxi) and were able to go places a large bus could not. We made numerous stops (including Hard Rock Cafe, Plaka, Temple of Zeus, Acropolis, the Metro [where they uncovered a lot of artifacts during the 2004 Olympics and those items are now on display], Parliament for the changing of the guard, which we could not have done with 50 people. For a little over $50, we saw it all - a significant savings from what it would have cost us going through the cruise line for a similar tour with less stops.
Port intensive cruises can be exhausting if you don't take time to smell the roses and enjoy your days at sea!