This wont cost you a penny. Give them some time off, time to relax and kick back, you can make a coupon for a night off with dinner, and you can make them dinner and some popcorn so they can relax and watch a movie after.
Lizardqg got it right! A few years ago my son made me a "flower bouquet" out of colored paper and tissue paper. Each flower had a different chore on it that he would do. All I had to do was pick a flower every day. It was the best gift ever!
A photo calendar...with pictures of you and/or siblings, or family as you have grown. Or a wall size photo of your parents when they were young, or a calendar with pictures of them as they have grown. I did both and they treasure it!