I visited Heidelberg in May, this year. The climate was just right, the cold almost left and it was a little warm.

It was a 2-day trip where we spent the first day in Mannheim and the second in Heidelberg. We stayed in an AirBnB where the host was super-friendly and was happy to offer directions and tips for our trip.
Mannheim was nothing compared to Heidelberg’s glorious castle and Old Bridge! The walk across the bridge is one that is a must-do if you want your mind to wander and get some peace while at it.
The walk up to the Castle Peak is just right – not too tiresome and not too light. The view of the whole city from the top is just scenic!
All-in-all, my opinion is that it is better to have a day trip from Munich to Heidelberg and skip Mannheim altogether. Do add it to your summer trips for sure!