I just got off the Imagination on Friday and I have to say I had a great time. With over 40 cruises under my belt this was my first four day cruise and the first in an outside cabin. We usually cruise in a balcony cabin.
I have to say that overall I was quite impressed with Imagination. It is an older ship but it is pretty good shape and well maintained. It is also a "fun" ship with more lively deck parties than you have on most seven day and longer cruises.
As Paul stated you have a wide range of food choices ranging from the buffet to the Main Dining Room. They also have the "Taste Bar". It was previously the sushi bar but now they offer a range of small bites of a variety of foods. There is still sushi in the buffet area.
The cabins are good sized. I was pleasantly surprised with our outside cabin on Empress Deck (7). It is almost as big as a standard balcony cabin on the newer and larger ships and it has all the comforts you need. There is a bit less storage but you don't need as much on a four day cruise. We did a back to back cruise and had clothes and "stuff" for a seven and were still able to fit it in without much problem. The shower was a decent size. I am 6' 2" and 250 lbs and I had no problem with the size.
The shows are good. I am not much of a production show guy so I did not attend but my wife liked it. I really enjoyed the comedy shows. The "adults only" shows can be a bit raunchy but it is no worse than an HBO comedy show.
In Key West I recommend just walking the streets and enjoying the architecture and ambiance. You don't have much time there so you can look into excursions if you have specific interests. I will say that the Hog's Breath Saloon has the best Bloody Mary's around.
I think I sampled one or two too many.
In Cozumel you have a large number of choices from ATV, 4x4 excursions. The snorkeling and diving are excellent or you can just kick back on a beach and take it easy. I recommend Nachi Cocum for a great beach day. It is all inclusive and they have a good beach, swimming pool, good drinks and EXCELLENT food. The guacamole is really good and I usually don't like guacamole.
Go: Keep an open mind, don't let the little things bother you and enjoy!!!!! It's easy to have a good time on the ship.