I don’t get the niece problem… it’s not a lack of itinerary but a lack of time. If I had a relative / friend visit … we’ll I wouldn’t care if we had to just sit in a Starbucks or even McDonalds or do the most obvious tourist stuff as long as there is time.
Ok I think I get it. It presumes you both have made time to do the activity.
My thoughts are … well there is timeout / lonelyplanet / TripAdvisor / meetup / city subreddits etc. so there is no shortage of ways to find cool things to do from 4 yr old to 100 year old!
I have a feeling that if successful a site like mentioned would devolve into a groupon or redbaloon though as it searches for more revenue.