My parents (mid-70's) recently came to visit me and my SO in Germany. My parents aren't in the greatest of health, and the visit took place in a heat wave, so fatigue and physical exhaustion were major issues. As a result, I'm going to use this opportunity to talk less about what sights were the best, and what worked well/badly for having an enjoyable time given these limitations.
The good:
Air-conditioned museums, or at least indoor shaded museums. In Stuttgart, my parents and I saw both the Mercedes-Benz and Porsche museums, and had a lovely time. They both have underground parking garages that connect directly to the museum, which kept heat stress to a minimum. We also toured the palace at Ludwigsburg, which was unairconditioned, but the interior rooms were still a reasonable temperature. If we had been smart, I think we would have planned more of this.
The Black Forest, particularly lakes in the Black Forest. We visited the Schluchsee, which is more of a place for locals rather than out-of-towners, but the higher elevation and heavy vegetation kept things cool, and we still managed a very pleasant time. It was rather a long drive to get there, however, and if we'd been smarter we would have stuck to areas of the Black Forest closer to Freiburg, which was our base.
Trains. Nothing nicer than stepping onto a form of transportation that will take you where you want to go without your actively having to think about it. Similarly, the days when my SO or I were able to drive/tour-lead, it was a major stressor off my parents.
Staying at my apartment. Indie hotels were rather hit-or-miss on temperature control and disability accommodations, but my apartment worked out fine (not surprising, I'd chosen it with these characteristics in mind.) Also nice, for people for whom life is tiring, staying in a house that already has all the toiletries/extra towels/food/laundry service at random hours, and home-cooked meals.
Ice cream. German towns are full-- FULL-- of small independent ice cream parlors ('Eiscafes') which are really, really good. We found ourselves stopping for ice cream once or twice a day, and it was never not worth it.
The medium:
Colmar and Neuf-Brisach. We spent the heat of the day in a French secondhand shop, and ventured out into downtown Colmar when the day was getting on and temperatures were starting to drop. We hit Neuf-Brisach in the evening when the sun was already well on its way down, which was good because the place would have been sweltering around midday.
Downtown Freiburg. It was fine because we weren't out particularly long and took plenty of ice cream breaks, but in retrospect the time spent walking around outside could have been better spent in museums (see above).
The bad:
A selection of small nature points in the vicinity of the Kaiserstuhl. We did this on one of the hottest days, which in retrospect was a not-great idea because the Kaiserstuhl specifically is reliably the hottest area of Germany. Live and learn.
Changing cities to be our home base every few days. In out case, we had good reasons behind it (wanting to see relatives), but, of course, the travel and hotel changes ate up quite a lot of time and was rather tiring. In the future, it would be nicer to cover less ground but explore a single area more thoroughly.