I went to Heidelberg after asking different people at Wikimania what to do with a few days to spare in Frankfurt. Several agreed that I should go to Heidelberg, to see it.
Heidelberg is a regular German city (I don't know, but I think so). It has the oldest university of the nation and lots of students. It's also a tourist hell.
The Alte stadt is nice to walk through although, again, it's full of tourists and shops for them. Disregarding this, both the Castle and the old bridge, being the two main attractions, were very nice. I didn't spend any money to go into the castle or follow a city tour; since I paid the extra money to go to Heidelberg I decided to be cheap while I was there.
Even though I didn't go into the castle I went on the balcony and saw all of Alte Stadt in a beautiful view.
Which brings me to an obvious point: It's ridiculous to even set out on a travel to see cities and countryside like this without bringing a camera. Every ten minutes I see something I want to shoot, either to help me tell the story when I come home, or because I think Wikipedia and the free content sphere doesn't have this picture yet. And, at Wikimania I learnt to use flickr -- only that I can't use it because I have no camera. I have lots of Stuff to buy now.