Adding "and a half" to a noun is a slang way of adding emphasis, a device that is pretty popular and common in AmE. A treat and a half just means a really nice treat, but the expression can be applied to many nouns. For example, if my friend has a dog that is very big, or outstandingly canine in some other way, I could say "That's a dog and a half." However, I recommend that learners avoid it. As with much slang, there are subtle nuances governing when it is appropriate and when it is not.
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