Christmas present for dil

Definitely go for something that involves time and not money - the babysitting while they go out is a great idea (unless that's something that you do regularly anyway). You get to spend time with the grandchildren and they get some adult time together. Win-win.

Offer to cook a meal for them, or to take their car through the car wash once a month.

Or (tongue in cheek here) offer to de-clutter her wardrobe, if she has so much!

I do agree with your last sentence. However, if it’s usual to give her a present how about a massage, aromatherapy treatment, or similar at somewhere like Neals Yard, or a health club local to her, or similar type establishment. People who have everything often appreciate treats. You could also offer a special afternoon tea somewhere for both of them. Good luck with a decision.

I'm in the same position. I'm going to give my son and DIL a voucher for their favourite restaurant and a " babysitting" voucher ( I'll babysit) for a night of their choice.