D.K.
Kids lie for several reasons (kind of like adults)
1. They want to give you the 'right answer' - did you brush your teeth? Well clearly the right answer is yes :)
2. They want something to happen or have happened differently. This may be what is going on with the playground stories. They are a what if sort of thing.
3. To avoid punishment or a 'consequence'. And no they don't quite grasp the 'it's better if I just come clean now' theory. Of course most adults don't either.
So, first, stop giving him the opportunity to lie. It is a habit, the less of it he does the less of it he will do. You know if he took a bath or not, right. Instead of asking, say - 'it's time to go take a bath since we didn't do it earlier'. You can see if he picked up his toys, instead of asking - say - I see you need help picking up your toys, let's go do that.
I also think you need to stop punishing him or threatening to punish him. He likely says negative/angry things because that is what he perceives/hears. If you want him to speak positively and nicely, I would try to treat him with respect and model the positive behavior you desire.