Going to the Carretera Austral in Chile for a week late February. Stopping in Santiago for a few days. Any recommendations for a day trip? Considering Valparaiso or a trip to Wine country.

Depends on what you are interested in. Both Valparaiso and nearby wineries are doable as day trips, as is a trip to the Andes. Although it is now old, here is my trip report on Santiago from 2017: Trip Report 10 Nights in Central Chile and on Rapa Nui - Fodor's Travel Talk Forums

Quite a few options depending on your interests: Cajon del Maipo, Isla Negra, Casablanca Valley for wineries (we LOVED our extended lunch at the winery Casas del Bosque), Vina del Mar, Valle Nevado, Rancagua (stopping at Santa Rita along the way), Andes (town) with nearby Errazuriz winery, etc.