Since you are going on the MIracle ship....you will be able to enjoy the group called Polaris - they play electronic cellos and are simply amazing. We bought their CD and listen to them all the time.....That is about the best thing I can say about going on a CArnival ship....Much prefer Princess...
We have boarded in the past in SF and always carry our own luggage on board as long as it will fit thru the machine. The cabins are usually open to drop your luggage and go to lunch.