The Himmelsleiter, or 'ladder to heaven,' is the direct old stone climb from the castle side of Heidelberg toward Königstuhl. It is simple in concept and hard on the legs: hundreds of uneven steps rising through the forest above one of Germany's most romantic city views.
Use it if you want the landscape rather than just the funicular. The reward is not only the summit area but the gradual sense of leaving the Neckar, castle, and old roofs below you.