Well PG2, masses of mostly very sensible advice!! Even down to the lifeboat drill that all ships do at the start. There's loads more advice on Cruise Critic, Cruise co uk, and the like, for all manner of questions, both on board, and at assorted ports.
Saga offers a lot, but you do pay for it, Fred Olsen specialises in the more mature British customer, and they have smaller ships too (not everyone likes the floating cities being launched now). We liked Celebrity, and their close cousin Azamara offers terrific service, on small ships that can access smaller less obvious ports - '6 star service at perhaps a 5 star price'? Perhaps beware a giant ship that has to use 'tendering' to get people ashore and back, that's using the lifeboats as little ferries, and not everyone enjoys it.
On most ships you don't have to use the main dining room, there should be a buffet restaurant if you prefer, that can help if you don't want to attend any 'formal' nights (which some ships do have). In such a buffet, you could have 3 starters and 4 puddings, if that's what you'd like?!!! (etc). And if it's a ship that assigns you to tables, (this can happen if it's not 'anytime dining'), you might be unlucky enough to be near someone who drives you bonkers. If so, have a quiet word with the Head Waiter, and you can be assigned another table -- they get plenty such requests so are used to dealing with it.
You'll get a leaflet poked under your door each evening, listing all the next day's events, on a big ship there'll be more than you can take in! As many have said, loads to do --- or NOT DO, if you prefer! Also many have mentioned 24/7 food, you do NOT have to try and eat it all (some American passengers may not realise this?!) and one entertainer I asked said that to stop turning into a balloon on a voyage, he only ate lunch. The main meals in the MDR tend to have quite a few possible courses, but you don't have to eat them all, and portions are usually quite small, don't expect a transport cafe plateful?!
Ship's tours are a constant source of complaint, invariably VERY expensive. We rarely do them, except to get somewhere tricky on our own, or perhaps where the language is a problem (a Greek bus sign is a mystery!). Do your research, at many ports you can do your own thing, or maybe hook up with other passengers, another idea on here. There are websites to help you book local tours, and I recall on Barbados we had a hire car waiting by the quay.
What else -- the last day can be a shock, need to get packed the night before, so a bit of planning, and it's an early start to get b'fast, and disembark. Tips ('gratuities') - lines do this in different ways, it might be included in the fare, these days, but some ships may give you little envelopes for various staff. We almost always have left a little something for our cabin steward/ess, whatever the main tips process.
Oh, and someone said "wouldn't go again, environmentally damaging". Well, on that basis, nobody should have a diesel or petrol car, nor go on a plane. The cruise lines all have website sections detailing the considerable improvements they've made!
