Agree- best to book dining and excursions on line- watch for sales- check often- especially around holidays. When on board, with a dining package, go to a restaurant representative- they are typically sitting outside restaurants or go to any specialty restaurant to book times/dates.
If you have anytime dining, never a problem. Wait on line at the selected restaurant and request a table for two- with fixed dining, as said above call celebrity or if you are dealing with a TA ask them to handle on your behalf.
We always take as much luggage as we want depending upon the lenght of the cruise- we store our suitcases under the bed after unpacking.
We typically do not bring snacks- food always available and junk food snacks are sold in the shops.
Typically at each port, you have a pretty long walk to the main area- you can get a taxi if you want to go to a specific location. It varies by port.
Unlike Royal, no show reservations are necessary, except for the comedy club.
We did a b/2/b on the apex last January and absolutely loved it. It is a great ship. Don't miss dining in Eden.
Enjoy your cruise.