If they've never been- I'd stick to the free museums, walking along the thames, see London bridge and maybe a walk in one of the large parks- but depends where you are staying? I think Big Ben is still covered in scaffolding, so I'd miss that.
If you've been before and looking for a bit different- get the river cruise (there is a regular, commuter one with no commentary which is cheaper) and visit Greenwich. There are markets, maritime museum, cutty sark and a fan museum! You can also take a trip on the emirates air cable car.
If you are there on a sunday, get out at shoreditch overground (or Bethnal Green tube or liverpool at tube) and walk around Columbia flower markets and then Bricklane market. The Bethnal Green end is more bric-a-brac, 2nd hand clothes etc, and the section in the middle of Bricklane- the upmarkets and around the curry houses are more bespoke stalls with handmade soaps, prints, jewellery, clothes etc.
If you go for a curry there, I'd only recommend preem and prithi (one has shut but I can't recall which one! they have the same owner though. Always had a good meal there. My husband did a cooking course in their kitchen once and said it was very clean.