The Barcelona Aquarium is really 35 aquariums in one, adding up to the world's largest Mediterranean-themed aquarium. A special treat is a 80-meter (262-foot) transparent tunnel in the ocean tank that allows visitors a unique view of what's it like to swim with the fishes. At the harbor end of La Rambla, it's a good spot to begin - or end - a promenade. .

aquariumbcn.com