The best breakfast in Kanchipuram is not in a hotel. It's in one of the small, no-menu tiffin restaurants that open around 6am near the temples and bus stand, fill up fast, and wind down by 10. There's no English signage; you sit down and get what's being made that morning.
The standard spread: idli (soft, steamed rice cakes) with sambar and coconut chutney, vada (fried lentil doughnuts, crisp outside and airy within), plain dosa or masala dosa with potato filling, and upma (semolina porridge with mustard seeds and curry leaves). All of it costs less than a cup of coffee would elsewhere.
The Kanchipuram idli — a local variant spiced with black pepper, cumin, ginger, and sesame oil — is the specific thing to look for. It is denser and more robust than the plain hotel idli and is closely associated with the city; the recipe is said to have originated as temple prasadam at the Varadaraja Perumal Temple. Ask specifically for it in local restaurants; not everywhere makes it.
Filter coffee arrives in a small steel tumbler resting in a wide steel saucer (davara) for cooling. It is strong, sweet, and very good. Coming in off the street for this alone is worthwhile.