You will probably not get your baggage until about 9 am. At this point, just walk outside the baggage area and grab a taxi which will cost you approximately 30 Euros to the port. This means you will probably get to the port about 9:30 and you will be able to immediately drop your luggage. Than, assuming you can handle your carry-on, you would be free to explore Barcelona. You can walk from the port to the Ramblas in about 25 min or its a short taxi ride (about 10 Euros). There is also local bus service from the port to the Ramblas area. Your other option is to simply wait at the port until they allow you to check-in and go to your cabin. For most of the cruise lines, you will not be able to get on-board much before noon. Over the years, we have learned to use backpacks for our carry-on which allows us the freedom to explore with our carry-on safely on our back.