In the very first act of Part 1, Hal says:
Yet herein will I imitate the sun,Henry IV
Who doth permit the base contagious clouds
To smother up his beauty from the world,
That, when he please again to be himself,
Being wanted, he may be more wonder'd at,
By breaking through the foul and ugly mists
Of vapours that did seem to strangle him.
isn't about a young prince maturing into a king, as I've often heard claimed; Hal had his plan in place right from the very start.
So, when this loose behavior I throw off
And pay the debt I never promised,
By how much better than my word I am,
By so much shall I falsify men's hopes;
And like bright metal on a sullen ground,
My reformation, glittering o'er my fault,
Shall show more goodly and attract more eyes
Than that which hath no foil to set it off.
I'll so offend, to make offence a skill;
Redeeming time when men think least I will.
Hal is using
Falstaff, using him to make himself look better. Falstaff is being set up; he's the "sullen ground" that will make Hal's "bright metal" shine even brighter. That makes Hal sly, conniving, and manipulative. I think we're meant to be amused by Hal's cleverness instead of taking it all too seriously, but I would never buy a used car from him.