I know Heidelberg is on every tour route, and always teeming with tourists, but I never miss a chance to visit when I'm visiting family in Germany. I love the Altstadt, with its romantic nooks & crannies. I love the bridge on the Neckar, & walking on the far side. I love the castle & the view it affords over the university & the Fußgängerzon. I love going for the Christmas markets tucked into every little square.
And I never miss stopping at Grün & Finck, my favorite little shop on the Hauptstrasse. The walls are lined with cases just like in a bookshop, except instead of books they're filled with bottles of the most delicious liqueurs. Every fruit imaginable, & not the icky syrupy sort you find here. I especially like the Bratapfel (baked apple), which is like apple pie in a glass. They also have more herbal flavors, spicy flavors, even oils & vinegars & honey. In winter they offer Christmas liqueur, in seasonally decorative bottles as well as their normal pretty bottles. This one tastes like Christmas in a glass!
There are so many flavors it's hard to choose, but they'll happily give you samples. The only problem is how to get all those bottles home with you, because you'll want some for yourself as well as gifts.