One thing you must consider when your companion is age 80+ is availability of medical services in case of emergency. Also you'll need to do less walking (and public transportation that involves walking and steps and standing such as lots of subways) and take more taxis or private driver, depending on destination. You also have to pace a trip less frantically than you might do if traveling solo or with friends, and avoid tiring overnight train rides and moving locations every night or so. All of that is harder on the aging body. You don't say how long you have, but I'd suggest Japan, or perhaps Hong Kong + Thailand. These places have a lot to see that can be done efficiently, have different food options available for the picky, and good infrastructure. Also, I would try to go on this trip in May, early June at the latest. July and August get steamy and rainy most places. Just read up on local holidays and try to avoid the major ones for your destination.

BTW, a big ^ for doing this sort of trip with your grandpa, while you can. Lots of young adults wouldn't even bother.