The phrase 'to something really special' is a preposition phrase (PP) functioning as one of the complements of 'treat'. This verb takes two complements: for example, 'I treated [my family] [to some ice-cream].' The PP headed by 'to' is the second complement of 'treat'.
Internally, 'something really special' is just the same as 'some ice-cream', that is a noun phrase (NP) which is the complement of 'to'. This NP is headed by the pronoun 'something'. Pronouns like this are unusual in that they can take adjectives, but the adjectives follow them: 'Something funny happened last night.' So in your sentence, something special is what you're treating your taste buds to. And it has, additionally, an adverb modifying 'special'.