How about making up 'DIY tokens' on your computer eg 25 hours of painting, window washing, car cleaning, weeding etc. Then print them off and give them to them in an envelope - totally free for you but allows you to help them out with jobs that need done.
Have they got a lot of family photos that need sorting out? It is a nice gift to organise the photos and put them in a (not expensive) album. Don't let them know that you are doing it - surprises are always nice!
If they (and you) are sentimental people then something which is totally free but a really nice thought is a memory box. Decorate a shoebox or similar and then fill it full of little keepsakes which chart your lives eg school reports, written out memories of your most special xmas, photos of pets and previous homes, birthday cards that you may have received from them etc etc etc. It can have absolutely anything in it and it is lovely to sit down with people you love and talk them through all the items. People forget socks or chocolates but a present like a memory box is totally unique and says thankyou in a really special way.
Good luck.