Aquarium de Paris - Cinéaqua (combining an aquarium and a two-screen cinema) is a wonderful attraction and a key element in the renaissance of the once moribund Trocadéro. Children in particular love the shark tunnel (home to 24 toothy harks), and the petting pool (bassin de caresses) where you can stroke friendly sturgeon who pop their long funnel noses out of the water. Also interesting (though less theatrical) is the section on the River Seine, showing the sorts of fish who still survive in Paris' river despite the pollution.
Many people baulk at the admission fee, but you can easily spend a long afternoon here, watching cartoons in the cinemas and observing the sealife. Even very small kids are catered for, with glass walls that touch the floor so that the little ones can see inside without mums and dads picking them up. On Wednesdays and weekends, there are special children's shows too.