Can you produce activated carbon from toast?
Is toast a good source of carbon is quite a broad question without considering the type of toast we're referring to. Even if do stick to a particular toast, what makes a good carbon source sounds a little too vague.

As a side note, recent researches have led to the production of a low cost multi-functional carbon foam from bread. [Source]

As a rough estimate, let's consider we're talking about white bread. 100 grams of white bread consists of 49 grams (i.e. approximately 16%) of carbohydrates. [Source] For the sake of simplicity, let's assume all of the carbohydates to be monosaccharides, specifically glucose. (This definitely isn't the case). The molecular formula of glucose is $\ce{C6H12O6}$. If we calculate the percentage-by-mass composition of carbon in glucose, we get the value as 40%. 40% of 49 grams is 19.6 gram, or in other words, bread consists of (very roughly) 19.6% carbon.

I'm unsure of whether or not that makes it a good source. Reading this article tells me that a rough estimate for the carbon content in wood (it is about some conifers but nevertheless good for rough estimates) is about 45–50%.



The production of activated carbon is undertaken generally in two ways: [Source]


Physical activation: It requires an inert atmosphere of nitrogen or argon along with very high temperatures (600–1200 °C).
Chemical activation: Strong acids, bases or salts and a lower temperature is required (450–900 °C).


Theoretically the chemical method seems appealing but in a practical sense it is not advisable to carbonize materials in a home-kitchen as the fumes evolved contain carbon-monoxide which is extremely toxic.