I think: They both have different plans to kill Batman; arguing over who's plan is better. The Joker will then tease her, she'll get annoyed, they'll have an argument then they'll end up making up... and making out :P
I think Myers_fan has a point. dXcFan14: Harley isn't really the brains of the operations, mostly sponging off of Joker to think of a plan and then going out and doing it to appease her "Puddin'."
I think they try and form a group in there spare time. Finding the fiercest foes of Batman and the city to throw at them using numerous different ways to destroy the Bat. For example using Mr.Freeze to create a snowstorm and other people like Bane to create a diversion to distract the Bat and keep him busy while the city freezes. and if that fails pull someone else in like Killer Crock. There are plenty of ways to do this. BUT if there aren't planning they are robbing stores for the heck of it and spraying graffiti.