We just cruise the Grandeur to Cozumel in March and found the seats in the theater were very comfortable. Yes, there's a place for your drink in the armrest.
Incidently, they just brought a new show on board and it was excellent!!!
1. Get to the theater early for the best seats.
2. Take time to explore and enjoy all the artwork on the ship. There's a great statue of a man "sleeping" with a newspaper on his head in the library that suprised us. Lots of other great art as well.
3. Look down on the atrium from the internet room in the evening. This turned out to be one of my favorite photos I shot.
Explure the ship and have fun!!!