As long as you are there, stay the month and travel around. The whole pacific coast is nice. Scenic vineyards. Atacama to the north. After time in Argentina take the opportunity to eat true fresh shellfish especially in the mid-south Bio-Bio region. Valdivia waterways feel like Seattle and coastal waters of Washington state. Valparaíso and the other beaches of that Santiago area will be filled with summer travelers from Argentina.
The Chile owned bus companies and smaller transport are easier to find at the Mendoza bus terminal. Not found on platforma10
You bring back memories
Vineyard tours in Casablanca, on the way to Valpo. Or around Puente Alto, before going up to Valle del Maipo (Go all the way to the glacier if you can, I've never been that far).
Don't be fixated on the beaches (blazing sunshine in summer, ice-cold Pacific water), go to the lakes in the south (Lago Ranco, white sand beaches, Lago Villarica in Pucon, black sand beaches), warmer than the sea.
Travel by bus, there used to be direct routes from Santiago (Vitacura) to Lago Ranco (Llifen).
A day or two in Valdivia would be nice, drive to Curiñanco, or if you're adventurous, take the "balsa" with your car to Corral and drive south. Be prepared for dirt roads, dust, blazing sunshine and unpredictable weather.
Drink cidra (a much more egalitarian product than in Argentina, the best comes in a recycled Fanta or Coke bottle). If you're at the "muelle" when the fishing boats come in (like in Los Molinos, near Valdivia), make sure you have your lemons and your box of cheap white wine and enjoy your seafood fresh from the sea, sitting on the pier.