Other than the general physical (walking and others but need to be careful: falling and breaking bones is a significant cause of loss of quality of life) and me

Other than the general physical (walking and others but need to be careful: falling and breaking bones is a significant cause of loss of quality of life) and mental (sudoku, Jeopardy, reading news from Internet, learning a new language) I would say the most important thing is to routinely be in contact with them using remote video calls. This serves two purposes: (1) Keeps them connected, mentally alert and feeling valued and (2) it will provide you a quick way to assess their condition and notice and red flags early.