Munnar surprises first-timers because it does not feel like Kerala at all. Pack for the altitude and for your season, and the tea hills are easy. Pack for the beach and you will be cold.
Layers beat bulk: a fleece or light jacket for evenings, even in March. Trekking shoes with real grip, because estate trails turn muddy after even light rain. A small daypack, a reusable water bottle, and a headlamp or torch if you are doing any sunrise viewpoint trips.
A rain jacket or poncho, not an umbrella: the wind on viewpoints makes umbrellas useless. Waterproof shoe covers or sandals you do not mind soaking, a dry bag for your phone and camera, and extra socks. Trails close and reopen with the rain, so keep your plans flexible.
A proper warm jacket. Mornings can drop toward 5 degrees, and the wind at Top Station cuts through a thin fleece. Gloves and a beanie if you plan a sunrise trek, plus lip balm and moisturizer: the air is dry.
Sun protection first: cap, sunglasses, sunscreen. The sun at 1,600 meters burns faster than on the coast. Light layers for the evening, when the temperature drops quickly after sunset.