Eve, we have never been on a 'big' cruise ship but we have done the Rhine cruises many times, in fact we are off on our latest on Boxing day this year.
We have upgraded to a Prom deck cabin so we do not have to go up and down stairs to our cabin, as the stairs can be quite steep.
The more you pay, the bigger the window in your cabin, from a porthole up to a full picture window. Lol.
Mind you its not really expensive any way, we are away for 4 days, breakfast, 3 course lunch, 5 course evening meal, entertainment, and travel to and from Germany. even with the upgrade for the cabin it has cost us £299 each.
The food is fantastic, we have never had anything to complain about, in addition to the meals, they bring Tea/Coffee into the lounge mid morning, then in the afternoon, its hot drinks again with pastries and cakes.
The only extra that you have to budget for is your bar tab.