Barcelona

There's very fast train service between Barcelona and Madrid, so if you're a big fan of art museums, Madrid is doable. Otherwise, Barcelona has more than enough to keep you busy until your cruise begins. The day you arrive after an overnight flight, you may be too brain-dead to sightsee, anyway, which would leave you only two days.

You need to get hold of a guidebook covering Barcelona and decide what you want to see. Barcelona has a lot of sights for which you'll need to buy tickets in advnce (if you're interested in them).

With only 3 days before the cruise, stay in Barcelona. It's a great city. Madrid is another great city worthy of a 4 day minimum stay.
We just returned home from visiting both.

I agree with others about staying in Barcelona.

The fast train to Madrid takes about 2.5 hours each way, which is a lot of train time for a day trip. With just three days, I suggest you stick to one city. If you don't have your flight reservations yet, you might consider flying into Madrid and spending most of your three days there, with the train to Barcelona the evening before your cruise (assuming that it starts there). But, as noted, you should have no trouble filling those days in Barcelona.

Thanks acrevan, David, Bob and Dick! Your answers were helpful. We decided to stay in Barcelona instead just to soak up Barcelona. My hotel is 15 mins away from the port. What are place I can visit, or eat in Barcelona once we arrive?

Omygosh - the choices are endless, both for activities and for food. My favorite site is the official city tourism website. The TI has its own tours and sells (sometimes at a small discount) tours by other operators. Runnerbean is one that many (me included) recommend.

If you like art museums, the MNAC and Miro are top-notch. There are mixed reviews on the Picasso Museum, some because it's always crowded, some because the collections are less interesting than at other Picasso museums. If you are huge Picasso fans, buy tickets in advance. Many of the Gaudi sights require advanced purchase, as does a tour of the Palau de la Musica (a personal favorite).

Three days isn't a very long time to spend in Barcelona, but depending on your dates you might be in luck as the city's annual La Mercè festival takes place from September 20–24 with lots of cool activities (castellers = human castles, correfoc = firerun, etc.).

If not people typically hit the main Gaudi-related sights like:
Sagrada Familia
Park Güell
Casa Batlló and/or La Pedrera

Beyond that the FC Barcelona museum at the stadium is popular (if you like soccer) as well as the Picasso Museum (if you're in to modern art).

I'd suggest buying one of the tickets to the hop on / hop off buses on your first day so get your bearings and see some sights.

Then maybe a walking tour on day two – there are some good free ones as well such as Sandemans (they rely on tips and also hope to sell you other tours).

Then the rest would depend on your interests – if you're in to food, maybe a food tour or a cooking class. If you like modern art perhaps the MACBA modern art museum, etc.

By the way La Rambla is fine to walk up and down but I wouldn't eat or drink there – none of the locals do.