I'm not sure how often they change the menus but to get an idea of past menus, go to www.cruiseclues.com and click on "Grand Princess" on the left hand side. All sorts of links will pop up and some have scanned menus from past cruises. I've found all sorts of wonderful info on that site.
Navigator of the Seas Jan 06
Grand Princess Jan 05
Constellation Jan 04
On our cruise with the GP last year - we found the quality of the food to be spotty at best. Some items were very good - some were not good at all. I am not too fussy - as I am also happy at a BBQ and was probably one of the few individuals to have nice things to say about the GP BBQ at the Princess Cays. The pasta was not very good, and the "beef wellington" was very dry - perhaps recylced prime roast. On the other hand, the alternative steaks were very good, and the alternative cheesecake was very much to our liking.
We also found that breakfast in the dinning room was an experience to be avoided. There were very few people there - the menu was not extensive, and the food arrived ice-cold after a long wait. Better to grab breakfast at the Horizon buffet.
Although I did not ask for another main dish, one can ask if the food is not to one's liking. I am going on another cruise this year with RCCL, and if anything is not to my liking - I think I will not be bashful and ask for a substitute.
Have a good time and don't overlook the free mini golf course. It doesn't look like much - but it is a riot - especially when in rough weather!