Our ship is scheduled for a refurbishment in April 2023, so until those details are released, we don't know what the cabins will look like. However, the information below should be correct.
Staterooms onboard Serenade of the Seas are nearly all the same, with the exception of window choice. Inside cabins have no window, but a large mirror instead. Outside cabins move the bed beneath a large picture window. And balcony/veranda cabins have a sliding glass door with a private balcony with chairs and a table.
The beds are king-sized, and can be split into two twins depending on preference. A screen provides satellite TV channels and information about our voyage. You can also use it to check your balance, sign up for excursions or order room service.
As there is a lot of time at sea, this might be the cruise to spring for a balcony. It's not that much more money for a lot more space.
The bathroom has an adjustable shower and typical toiletries. There's a safe for valuables and a hair dryer.
Electrical outlets are US and European, plus there should be USB charging ports with the refit. If you have a lot of devices, consider purchasing an inexpensive US to European converter (will not change voltage, but will provide power to most chargers. Check your charger's rating to be sure.)