Oh! We are in the complete opposite boat to you!!! We go to Germany a few times every year, and right now we're trying to decide whether to forgo the next few trips to save money for a trip to WDW for food and wine next year! I hope you have an amazing time!!! Here are my tips though:
Trains in Germany are widespread, fast, reliable and fantastic value for day tickets - I know you have a car rental but it if you are doing your own food and wine (or beer!) day out I'd leave the car behind so you can make the most of it.
Does where you're staying change a tourist tax? If so, you might get some discounts like free public transport. I've never stayed in Heidelberg but its not uncommon in other parts of the country.
Is your accommodation self catered? Lidl is a great discount food store. There will also be plenty of bäckerei if something small like a pastry or filled roll will do you for breakfast.
I'm not sure I'd bother leaving Germany, except maybe for Strasbourg as its not too far away, but really Germany is a fantastic and varied country with lots to see and do. If you need to scratch a theme park itch, and you do stay stay a night in Strasbourg, one of Germany's best theme parks is nearby. Europa-Park opens for its winter season on the 24th November.
Have a fantastic holiday!