there is an easy way!!!
ok you can leave your luggage there but,
why no boarding the ship early and then go and explore?
So you know you can board ship,
* Carnival Destiny at 11:30 am. you can do all the check in at that time but, you will have to wait on port for 12:30-1:00 pm. to enter ship. By that time there are no lines, if you try to board ship at 2;30 pm you will have to wait a long time, in line and by the time you enter ship it will be a to late to see much of old San Juan!!!
* Remember if you enter the ship early you can make all the arrangement of your dinner table seating, even can change your table number if you wait until 2:30 pm there is no options for you and changes will be more difficult.
* Adventure of the Seas (RCCL)is almost the same case, you can go to port at 11:30 am and will board ship by 12:30 - 1:00 pm. no problem
why this way is better?
Well you can hit the buffet and take a shower before going to San Juan.
Also your luggage will be at your cabin early.
I remember that I had my luggage by 3:30pm so no trouble at all I even had my tours on my cabin during sunday night.
If you are going to buy tours using carnival tours do it before boarding on their site!!!
If you need any advice please let me know,
I am from Puerto Rico and I already make some posts about my beautiful island, Carnival Destiny and their ports of call.