We've been to Barcelona four times and La Sagrada is the only place we have gone all four times. Well, we did walk the Ramblas and go to the Market all four times.
I usually figure if you have to ask, its too close. I am risk averse and would rather be waiting on line at the port than worrying that my flight is an hour late.
We went to Pompeii on our own and were glad that we hired a guide. The guide put a lot into perspective above what we could have got from a guide book. There is a lot there that just looks like ruins.
We have been to Barcelona four times and we still have things on our list to see. Each time, we go to La Sagrada to see how much progress has been made. Also, not on the HOHO tour is the palau de la música.
Recommend that you be on deck for sail away. The SOL will not be on the same side of the ship as the Manhattan sky line. You may have to get up early but the return trip up the Hudson is worth going out for.
One reason there is a continual flow of taxis is that they are coming to drop people off for the next cruise. Like a previous poster we saw taxis continually come into the port.