Great thread!
We have done the Med twice now with
DCL and lots of other times with other cruiselines.
We too would book an excursion back in the early days, but then realised that the cost for two of us, then for three when dd came along, was more than hiring a cab. We have been doing our own thing with our dd, since her first cruise at just over a year old.
What you need to remember is that all the ports are frequented by large cruise ships almost everyday of the week, the cabbies know the fares they should charge (and those they can get away with!), they also know exactly what a visitor wants to see, it is not a problem with any of the stops to get off and find a cab.
My advice would be to do a little planning, pick out the important things to you and negotiate, if that is what you want. My first criteria is look for a cab that is 'safe' and reasonably recent. I then ask the cab driver a few questions, to see if he has a reasonable grasp of English!! Asking do you speak English is not a good one!! Don't be worried to walk away or negotiate, that is part of the game and will be expected. Have a figure in your mind and stick to it. We usually work out the cost of the tour and keep under that - although you have to remember that the ability to stop when you like and move on again, beating the queues, has a value!! I can't stand waiting for people to get back on a bus after a comfort stop, or unnecessary tastings of horrid wine in a shop that doesn't interest me!
Having said all the above. We often do the ports without a cab.
Malta is easy, you walk up into Valetta (it is a bit steep though), which is a great town interesting, old and good shopping! However, to see Mdina and The Blue Grotto, you will need either a bus or a cab. Buses are at the entrance to the walled city of Valetta, there are lots, clearly marked. However, for time, I would suggest a cab, which will cost about 40 euros to do all the main sights. To get a cab in Malta is very organised. When you get off the ship, walk along the quay to one of the small booths and decide which trip you want - there is a sort of 'tour menu'. You then take the ticket they give you, to the taxi queue and off you go. If you don't want to see something, say, if you want longer somewhere, ask. You have the cab for about four or five hours, we did the Blue Grotto, Mdina, the fishing village and then back to Valletta, he parked up and waited for us in each place. Some times we were finished within 20 mins (fishing village) and wandered back to him, another time we were longer (Mdina, we had a lovely lunch). Take your swimwear to the Blue Grotto, just waiting for the little boats to take us around to the caves, made me want to jump in the crystal clear water.
Naples, cabs will be out in force, there are also people carriers too I noticed, for larger groups. Naples can be a bit hectic, the drivers swamp you as you come off the ship, and it is a bit of a pain negotiating, as they come across quite determined, Remember this is normal for them, they do this everyday and it is their job. They all know the price they want - have a look at some of the previous cruises to get an idea of what you should pay, cruise critic is a good source. Again, it will be all the usual places, a standard tour, unless you have something specific. In which case, if you don't speak Italian, write it down, or have a guide book open to the page. We have had a couple of fantastic drivers, who have been fun informative and helpful. I don't think in all our cruises (14 in Europe), we have had a really bad one. If you do, take the number of the cab and let someone know.
The walk to the train station isn't a good one, it is a very busy road, or you can walk through the buildings - it isn't an exact science unless you have a good map, we ended up getting a bus this time, but you need to buy a ticket before you travel at a newsagent. When you get on the bus, verify it, by clipping it with the yellow machine on board. I would suggest you get a cab there, (I believe there is a bus just outside the port, but we had walked over the main road and into the streets to get ours). From the train station you can go down to the line that takes you to Pompeii, or Herculaneum. Herculanum is great, smaller than Pompeii, much easier to understand and less busy. It is a five minute walk down the hill to the sight from the small station. Outside the station in Herculaneum on the left, is a company that will take you up Vesuvius. This is just amazing. You don't need to book, they go every hour or so. Go into the office, get the time of the next trip up - we have done this twice now and each time a minibus has been there ready to go. I can't remember the cost, but it wasn't a lot of euros (maybe 15 or so each). The bus takes you up the car park on Vesuvius and you walk the rest of the way. It is a steep cinder track (don't wear sandals, the stones get in them!!), takes about 30 mins. to get up. Our six year old did it no problem, with a bit of encouragement. My otherhalf carried her up as a two year old a few years before! I was amazed at how excited she was to be up a volcano!
Coming down is easier, there is a little hut at the car park to buy gifts, drinks and ice creams. The bus took us back and dropped us off at Herculaneum which was good and then he said he would pick us up again in a few hours - he charged us 5 euros for this little extra jaunt, we could have walked it, but it was very hot and well worth the extra euros.
Previously in Naples we have got the Hydrofoil, or ferry over to Capri. This is just a short walk from where the ship comes in and much cheaper than doing an excursion. Capri is a tiny island, walkable, but steep in places. Lot of little shops, stunning views, a perfumery and cafes and restaurants, very pretty. Although I have been to Sorrento many years ago, I have never done it from the Port, but I believe you can get to it easily. I think we may do that this time.
I will post our experiences for the next ports shortly and don't forget, a good way to cut costs, especially for a couple on their own, is to join up with another couple that you may have met. We are still in touch with two couples who we met in a cab on a cruise!!!
Hope this helps.
Kate
Then we had lunch at a little cafe near the port that was very yummy. 
We were really hoping for more beach type options there. I think after the hectic beginning to the week we will just want to VEG somewhere under an umbrella with a couple of adult beverages!! I have heard that there are some great beaches there...disappointed that DCL will only offer one choice for that. So I am interested in hearing what anybody can tell me about Mallorca!!
