M.O.
I think the best things you can do is DOCUMENT their lives.
You cannot change their health situations, limitations in driving/coming to see you, or how long they live.
When you are with them, try to focus on enjoying their company. Take lots of pictures, especially of your children with them. Video your favorite things...for example, my dad used to have a beautiful garden he liked to work in (I have no pictures of it or him doing his hobby), and he had a GREAT laugh, I regret not having video of him smiling and laughing.
Try to plan a vacation WITH them, if possible, so that you build those memories now, take pictures and can later recall experiences having fun with them. If they are not willing/able to GO on vacation, then plan a staycation where you can go to the zoo, plant flowers (you can put some in your yard to enjoy for years to come and know them as "grandma's daisies"). For every Father's Day (or close to it) our family takes "the dads" to a minor league baseball game. It's fun, relaxed and MUCH cheaper than a major league game. Grandpa buys the kids WAYYYY too many treats, we snap pictures and all enjoy a "picnic" on blankets near the outfield. It's a great "tradition" we look forward to every year.
Finally, they do have "memory books" for grandmas and grandpas to fill in. They ask questions like, "what did you like to do as a child", "how did you meet Grandma", "what was the happiest time of your life". It comes in a flashcard stack that is spiral bound or I've also seen little books too. I have learned so much about my own mother that I'd never thought to ask. We gave them to ALL the grandparents and asked them to fill them in at their leisure. I hope my kids have them to look back on.
Because we lost my dad early (my children never knew him) I keep a picture of him in each of their rooms. We talk about him all the time. They know he watches us and helps us...I have told my children that if they are mad at me or having a difficult time, maybe Grandpa can help them. Just because he's not with us, doesn't mean he's gone.
I am sorry that you are watching your parents health decline. I know it's difficult to see their health fail and them seeming to "opt out" of life. All you can do is offer...offer to visit, offer to plan a vacation, offer to bring the kids over. If you respect their choices, even tho you may disagree with them, you may have the opportunity to enjoy them more while they are still here as opposed to "fighting" their decisions/way of life.
Best wishes