On the northern side of the Neckar, you can walk the Philosophers' Walk (Philosophenweg), so-called because Heidelberg's philosophers and university professors were inspired by their conversations during the walk. Ha! Only if they were younger and fitter than most.
For normal older folks, the walk is a challenge, more than 50 stories high, much of it inside narrow high walls, covered in moss, steep step after step, eventually coming out above the castle (schloss). Along the way, there are rustic cabins and gardens, memorials, statues to writers, even a cemetery. But best of all, there are several viewing spots where a tired climber can recover and look at the most amazing views in Heidelberg.
Rather than give up and take the "easy" way back down, each stopping place inspired me to go on to the next.
Each narrow passage blinded me to the insurmountable heights above me, but each stop reminded me of the heights I had reached. Bypassed by people who were actually running up the steep passages and left behind by children who saw it as a lark, I was so proud when I reached the top of the top.
Rainy and overcast all day, the sun came out, lighting the whole hillside and bringing Heidelberg out of its shadows. An Aha! moment that looked like an Aha! moment.