I was also going to suggest discussing sex, in sufficient graphic detail. Like the time I took my teen difficult children shopping for condoms (they sell them in the supermarket here, on the shelves beside deodorant and toothpaste, the next aisle from peanut butter and Vegemite). I was loudly discussing the merits of flavoured vs unflavoured, warned them away from the banana flavoured ones. I also pointed out various people's comments on ribbed vs non-ribbed. We then paid for them at the checkout (very red difficult child faces) and once home I showed the kids where they were kept and welcomed them to help themselves. They never did.
Other things to discuss - difficult child's adoption. My sisters used to tell me I was adopted; I was really a little pig that had been adopted. Every time I raised a logical objection, my sister countered with a logical explanation.
"What about my tail"
"Oh, we had it amputated."
I don't have a piggy nose!"
"Yes you do - see?" and she pushed the tip of my nose up and dragged me to a mirror. "And you have piggy eyes, and you squeal just like a pig..."
At which point I demonstrated the squeal by telling, "Mum!"
Or you could discuss the bodies you have buried under the foundations. Or the bank robbery you're planning.
Whatever you talk about, it either needs to totally turn her off eavesdropping, or it needs to pique her curiosity so much that she HAS to ask you more, and in doing so, gives away that she was listening in.
It's like that wonderful joke about St Peter & God watching a priest who was so desperate to play golf one Sunday, that he called in sick and then snuck out the back with his golf clubs. As they watched him play, St Peter said to God, "Are you just going to let him get away with this? It is scandalous!"
God replied, "Don't worry, I am going to punish him really, really severely."
Just then, on the most difficult hole in the course, the priest gets a hole in one.
St Peter says to God, "You call that a punishment?"
God says, "Yep. Who's he gonna be able to tell?"
Marg