'You said it'd be just the thing' means that someone said "something will be just what is needed to cause/influence others to do something.
Fred: Putting in new boilers will be just the thing we need to bring in newlyweds. They'll be happy to have warm apartments for their newborns.
John: [a year later] You know those boilers, you said it'd be just the thing to get more newlyweds in our condo. I one year, not a one has signed a lease.