The best place for panorama views over the city is without a doubt Heidelberg Castle/Schloss. From the gardens surrounding the castle as well as from behind the castle walls the possibilities for perfect pictures are a plenty. The giant wine barrel in the castle is very fun to see and climb on too. If it's possible, do try to go when it's sunny. The rays of sunlight filtered between the lush trees make it an ideal place for relaxing on one of the many park benches.