Forget Smite. Consume gives Nunu all the true damage vs minions he needs. Max it second and its cooldown drops to 10s before CDR. You only need it once per wave vs the big creep, a snowball with some AP is enough for the little ones.
Nunu is fun because of his fantastic AP ratios. 1:1 on snowball, 1:1 on consume (plus, spellvamp), 3:1 on ult. With his haste to reposition, he wins any poke fight. Snowballs are like mini homing Nidalee spears, chunking down health bars.
His problem comes when he's confronted with tanky derps, and a snowball barely dents one of them for 10% of their health, because he can't do a great deal more. He also suffers against pushers who can sustain, because early on, spending cooldowns and mana to harass comes at the direct cost of the lane pushing against you.
I haven't experimented with AD or hybrid Nunu, but a 65% AS steroid paired with permanent ranged slow is worth investigating.