Assuming you arrive at noon it will probably be about 1pm before you get through immigration/customs and claim your luggage. The ride to the port will take about 20 - 30 minutes and you should be able to immediately board the ship. By this time is would probably be about 2 pm and you still would not have had lunch. The ship will be docked across the bay from the heart of downtown (The Ramblas) and it probably doesnt make a lot of sense to try and do a tour when you still have your luggage. Once you are finished your embarkation, you would have the option of leaving the ship and seeing some of the town. It take less than 10 min to get into the center of town from the ship (by bus or taxi) or you can even walk it in about 25 min. Barcelona is best seen on your feet whether with a guide or simply on your own. Its a very tourist friendly place with excellent public transit and taxis, so its pretty easy to see any site of interest. Most like to start by simply walking on the Ramblas, walking around the adjacent Gothic Quarter, and perhaps taking a taxi over to see the Sagrada Familia (Gaudi Cathedral). There is enough in this city for several days (this is why most try to do a pre or post cruise stay) so you would really have to pick and choose.