The Atlantic Sea-Park is one of the largest aquariums in northern Europe, built into the rocky shoreline west of the city centre and designed to showcase the marine life of the Norwegian Sea and the Sunnmøre coastline. It opened in 1973 and was redesigned and expanded in 2015, and it's excellent: the tanks are large, the exhibits are focused on local species, and the outdoor pool is genuinely impressive.
The park is about 2.5 kilometres from the city centre — a pleasant walk along the coastline, or a short bus ride. Allow two to three hours. The surrounding park area and the views across the fjord are an added bonus in good weather.