Hi Dave, and firstly welcome to the board & community.
Right Norway, well first question is which ports are you looking at, as although some towns are easily accessible from the port, others are not, whilst others offer very little in the way of interest.
The other thing to take into consideration is that the ports/towns can be quite small, so you may end up in very long queues if you DIY the day, a perfect example of this is Bergen, where if you get to the funicular before the ship tours do you will have an easy transfer up to mount Floyen, but if the ship(s) tours get there first, you may have to wait for ages for a space :o(
Places like Andelsnes offer little in the village itself, so you really need to either go on tour, or plan to spend a relaxing day on the ship. Whilst Geiranger, Oslo, Alesund and so on give you a host of options as soon as you step off the ship.
If you list the towns/ports/villages etc & what sort of things you enjoy, then we will do our best to give you some suggestions.
Alan & Katrina
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Having just done this myself, I can offer a little advice. The internet is a great resource. Often you can find reputable local tour operators by simplying plugging in 'Bergen, Norway tours', just for example. I would then cross-reference with TripAdvisor, just to see if other people used those same companies and what they thought. Then I would contact the companies and ask lots of questions, like how far are they from where the ships or tenders are, can you book in advance, will they take credit cards on the day, etc. And of course, as a newcomer to this board myself, I've found the folks here very helpful as well.
Here are some suggestions based on my own research: In Bergen, you can take the Citytours Sightseeing Hop-on/Hop-off bus. It hits all the major sites, including the funicular. They have a pickup right near where the ship docks, and you can pay on board with a credit card. The cost is about $23 (depending on the exchange rate at the time). It's not as 'intimate' (in terms of having your own guide) as the standard bus tours, but it's cheaper and you can stay a lot longer at the different sites. The price of the funicular ticket is about $13 (can also be paid by credit card) But as someone else pointed out in a different thread (I've read so many recently, I forgotten who...), you do run the risk of long lines when you get to the funicular if you get there after the cruise tours. But as the ticket is good all day, and the ship is there until 6, this probably won't be a problem.
In Geiranger, it's a little trickier. There's a company called 'GeirangerFjord' (StartPage - Geiranger Fjordservice AS) which runs a bunch of different tours. They are located literally steps from where the tenders are, and you can book online. We're booked on the 1:00 RIB tour and the 2:40 trip up to Mt. Dalsnibba. BUT, if you've been following my own angst-filled thread, there may be a timing issue. The bus tour doesn't get back until 5:00 PM, and the ship sails at 6. Princess has told me that the last tender should be going back at 5:30 (since their own tours come back then), but this could change at the last minute and so I would say book at your own risk. I thought about it long and hard before I decided to do it. (hey, maybe if we get a lot of people to go on this tour, the ship will be less likely to leave us behind!) The RIB tour is around $72 and the bus trip around $40. (Princess offers 2 excursions similar to these, but much more expensive AND you can only do one, since they overlap) The website also offers other options, so you might see something you like better. BTW, if it comes up in Norwegian, just click on the British flag in the top right-hand corner.
In Reykjavik, I found 2 highly rated (on TripAdvisor)operators; BusTravel (Iceland travel deals, Iceland bus tours to Blue Lagoon, Thingvellir and more) and Iceland Horizons (Excursions from Reykjavik, Iceland, with IcelandHorizon.is). Both offer tours of the 'Golden Triangle', which is supposed to be the 'don't miss' deal in Reykjavik. The only problem is that we don't dock until 10, and all the full-day tours (other than the one from Princess) go out at 8 or 9 AM. But BusTravel offers an 'Afternoon Golden Circle', which doesn't leave until 12:30 and is back in plenty of time for sailing. It does the full Golden Circle, but leaves out lunch, the miscellaneous stops that the full day tours make, and the shopping. As with everything else, you can (and probably should) book online and the cost was about $52. And they have a pickup at the dock where the ship is.
There are walking tours in Allesund, from the tourist center. And I'm sure the other ports have options as well, but I wasn't as successful finding those as I was in these 3 places.
All the companies I've mentioned have e-mail addresses and they will answer all your questions if you write them.