Phone beforehand, and say you are going to Tesco's, what would they like you to bring, and what meal are they planning tonight so you can pick up the ingredients for it.You go round to their house, say hello, and lavish attention on their oldest child.
Then, you go into their kitchen, wash the pots, clean the surfaces, mop the floor, load the washer and drier/hang clothes to dry, wash bottles if they are using them and put them in the streriliser.
Then, you prepare their evening meal for them, and leave it in the oven (If an ovenish meal) with clear instructions as to what time to take it out and eat it. Wash any pots you create by doing this.
Offer to take oldest child out for an hour. take child to a wide open space and let it run like a whippet for that hour, leaving it docile on return to it's parents. Have a pick up in the living room before you leave.
This, I can imagine, would be a fantastic present for a second time parent.