I highly recommend that you visit the BARCELONA AQUARIUM, located right next to Maremagnum shopping center, in the New Port... It's very big and modern, very well maintained and you will see some wonderful and rare species displayed there --- even PENGUINS! The entrance is a little expensive (I got a discount and still paid 9.90 EUR) but it's worth it, especially if you're going with children: they'll love it.
The visit isn't that long, but try to go there very early or by early afternoon (2-4 PM, when people usually have lunch in Spain) to avoid crowds, or you WILL spend a long time there -- making a line to try to get in! They also have a cute souvenir shop.
You can sail by the port in a swallow... "Swallow" (golondrina) is the name of a ship you can find at Por Vell (in front of Columbus Monument). The trip lates an hour, and lets you see the port.
Puedes recorrer el puerto en golondrina... "Golondrina" es el nombre de una embarcación que parte del Port Vell (frente al monumento a Colón). El viaje dura una hora, y te permite conocer el puerto.
You'll be able to see the sharks, fish, eels and other species swimming right above your head... while walking by the crystal hall that goes through the big pool where they live...!! It's really spectacular.
Barceloneta, the area with the harbour is known above all for the seaside, the harbour itself and the mare magnum, a shopping centre made by restaurant discos, discos and still discos.
In reality the Barceloneta is the old area where fishermen lived, and, if you just walk through the streets near the harbour, you'll find the old houses, smells and noises that the new olimpic area and the ramblas sure forgot.
This area is also full of delicious maresquerias(sea food restaurants) and simple bars where you can taste the best tapas in a really great atmospheare.
At the end of the Rambla de Santa Mònica is the majestic site of the Reales Atarazanas (Royal Shipyards)In the 20th century they were restored to house the City Maritime Museum.It is well worth a visit.