Lisbon's Parque das Nações is just a modern area of the city by the river with a stadium, shopping centre and large train station. For that I would only visit if you have ample time to spare however it is also home to probably the best aquarium I have ever been to! At 15.30 euros (when purchased online) is was a large portion of our budget for the day but so so worth it and that ticket includes the temporary exhibition too.
The permanent exhibitions surround a huge double level tank filled with sharks, rays and many other fish. Don't get over excited and hang around the first view point with the crowds as there are plenty of opportunities as you make your way round. Other highlights include penguins and seals!
It was such a great day out.