Heidelberg

The castle, of course. But just wandering around the old town is a super way to spend part of a day. Do some google searches about it and see if it interest you.

Heidelberg is quite pretty so I sure wouldn't go anywhere else. It can be rather busy/touristy, but still a very worthwhile and enjoyable place to visit. I would suggest the castle as well. It's pretty, interesting and offers great views of the old town. I think in five hours you could feel like you saw much of Heidelberg, if you have a plan (castle, river, old town) and a good map/guidebook.

I quite like the Student Prison as it is so unique. Visit the Aula while you are there. If you have time read what Mark Twain wrote about Heidelberg.
The Jesuit Church is lovely, simple white marble with gold and green accents. A bit different than many other churches.

That sounds like just the right amount of time for Heidelberg. It's a beautiful town but very touristy, which can get frustrating if you're staying longer.

My favourite thing to do in Heidelberg is to cross the river and walk up to and along the Philosopher's Way. It's a beautiful path with stunning views of the Old Town, including the castle. Not to be missed!