Haven't sailed on the Zenith yet, but will in 2007.
Being said. Lisa is correct. No clock in room only on phone..a travel alarm is a good idea, however you can also get a wake up call.
On all of the X ships I've been on there are robes for your use, and a hair dryer in the bathroom. Some of the ladies I travel with prefer to bring their own which you can plug into the vanity in the cabin.
As for formal nights, on a 4 night itinerary you will most likely have one.
The first night Casual. Second Formal. Third informal or casual and the last would be casual.
On my recent 3 night cruise on Mercury we had two casual and 1 formal.
For formal a dark suit is best...a tux is more fun. You can rent one and have it delivered to your cabin so he doesn't have to drag it around. Informal would be a sports coat and slacks.
To be honest you really don't need much. Maybe 1 small suitcase between both of you.
You each need a formal outfit, informal, and casual. a couple pairs of shorts , a couple casual shirts and sandals or casual shoes...
My recent 2 night cruise I only brought a carry-on and actually overpacked a little.
Less is more....with the exception of the evening wear.