i think you can show it to your attitude to them, caring for them.. ones a while call them just asking what and how their doing. asking whether they have lunch,etc. it really mean a lot to them.
for me. i occasionally asking they out just for talking, having lunch or dinner, seeing movie. and shopping.
don't forget about the special day, like their married anniversary, birthday. if they don't want a gift. you can asking dinner out and gather with the whole family. including their grandson and granddaughter,they would enjoying watching their grand.