You need SMITE, take it.

I've been playing Nunu for a while now and I still like to pick FLASH over other Spells, but some people like to use EXHAUST for an extra CC or TELEPORT for fast ganks.

MACHETES
Poacher's Knife If you are going to do Counter-Jungle
Ranger's Trailblazer If you are gonna farm and sustain yourself.

VISION
Sightstone If you want to take the control of the objectives or if you want to disable your enemy's jungler by warding all his jungle.

ITEMIZATION
Randuin's Omen Great against AutoAttack reliant champs like ( Vayne Tryndamere Master Yi) Also has an active that slows the enemys (It's always useful)
Frozen Heart Also helps with AA reliant champions but it gives you 100 armor without any health wich helps you with the Blade of the Ruined King 20% CDR which is always helpful and 400 Mana that helps you to stay longer in the fights/jungle.
Spirit Visage This is my favourite magic resist item because it has the passive that increases 20% any health regeneration (Works great with your Q Consume) It also gives you a good amount of health, hp/5s, Magic resist and 10% CDR.
Locket of the Iron Solari Is a team item, I would buy it if there's an area on effect ap champion such as Katarina with her R Death Lotus because it gives a shield to all the nearby allies and also it has an aura that provides 20 MR. It gives you Health, hp/5s and 10% CDR.
Banshee's Veil It gives you Health, Health regen after being attacked by an enemy champion, Magic resist and acts like a spell shield.

MAXING Q FOR CLEARING AND DOING OBJECTIVES
This allows you to invade enemy's jungle and clear fast the big one from the camps so they dont get the buffs and also it gives you the chance to do early drakes with your team or without them.
MAXING E FOR BETTER GANKS
This gives you a great CC for the ganks and a minimal cooldown besides a great sustained damage.

AMAZING SUSTAIN
GREAT COUNTERJUNGLER
SECURES DRAKES AND BARONS PRETTY GOOD
HIGH DAMAGE ON ABILITIES EVEN BEING TANK

BAD GANKS
IF THE ENEMY TEAM IS SMART THEY WILL WARD THEIR OWN JUNGLE
SLOW MOVING
CAN'T CARRY