If someone is Old and fat or Smokes and drinks, Good for them. I know you want to see them stick around for a long time, but at that age, they're happy just let them enjoy life.

Intive them to spend time with you.
best ways for family to lose weight and get out is with family.
for most grand parents it's their grandkids, for younger parents it's their kids.
intive them to the park to play catch or go for a walk to bond with you.
baking is another good idea, be interested in a recipe she's made and ask her to make it with you so that you can learn from her.
find something they are interested in and intive them to share that interest with you. that way you both get something out of it.