For nature & scenery, Petřín Tower & Hill delivers.
The combination of a leafy hill, a funicular ride and a climbable lookout makes this the most relaxed of Prague's big views, and far less crowded than the castle. The catch: it's 299 steps to the top with only a small lift, the tower view costs extra on top of the funicular, and on a grey day the panorama is muted. Best paired with a slow walk down through the orchards.