Our trip over was in the second week of September. Warm during the day, only cool in the dead of night when we would go out for some stargazing. Very, very smooth sailing. The QM2 has Massive stabilizers that make it the smoothest ocean liner in the world. Our weather was beautiful, maybe unusually so. No rain or rough seas. I wore short sleeves when I was 'casual' during the day. A long sleeve shirt and/or a light jacket at night.
Our trip back was in the middle of October. It was cool in the day and could get pretty cold in the night, especially with the wind. We had some rain and rough seas at first. I actually got a bit of seasickness, but not much, and only on the first day or two out of Southampton. I found that keeping food in your stomach helped me combat that feeling (easy on a pleasure boat).
Clothes-wise I am talking about what to wear between and after meals and events. If you are dressing for dinner, your path will not go outside unless you wish it to.
We had a 'Standard Oceanview' room on the way over. It was of course, not very big, but fine for a couple. About 200 sf. The bed seemed to be somewhere between a queen and a king. Very quiet. We had an inside room on the way back, about 175 sf. Not by choice, the tandem voyage booked the ship solid. But really, you spend a lot of time out of the cabin.
Standard Oceanview:
People did bring their pets, but they must be housed in the kennel. We did not bring any pets, but we enjoyed watching others visit their pets in the small visiting area on the top deck.
What did we do… Well, it took a day or two just to explore the entire layout. The passengers tend to be well educated or at least bookish, so they always book interesting speakers to give lectures. There's a ballroom for dances, A club for modern and disco. One of the pubs has jazz which I enjoyed a lot. They have movies each night in one auditorium. Oh, and this is interesting - They have the first ever planetarium at sea. There are four incredible films that they show in the planetarium during the voyage that you Must see. They are well directed and narrated by huge stars, Harrison Ford for one. I went to the gym each day. We also spend time in the huge library.
The Internet. My wife and I had to have access for business and for pleasure. I blogged for friends and family on the ship and in Europe. - Not cheap- I think we spent a grand shipboard for access on the combined trips.
You can be busy or you can relax and watch the sea. I encourage you to look into it.