You are going to hit the highest part of high season, so you need to book everything.
If you are starting out from Santiago, then drive south on Ruta 5 to San Fernando and turn off onto i-50 for Santa Cruz. This road euns from San Fernando to Pichilemu on the coast. Santa Cruz is the heart of the Colchagua wine valley. Book your lodging and ask them to book tastings atone or two vineyards. You can take a taxi to/from these as taxis are not expensive and the drink driving laws in Chile are very strict.
Spend a night or two in the area. You have a car so are not limited to hotels in the town. There are good restaurants in Santa Cruz. Plenty of small villages to explore between San Fernando and Rancagua to the west of Ruta 5. Peruse a map.
From Sta Cruz I would take the road to Pichilemu on the coast, perhaps for lunch on the way back or even a night's stay. It is a big surfing town. From there you can head back the way you came or continue on to the road toward Melipilla, a nice gently rolling road, where you pick up Ruta 78 back to Santiago.
Roads will be crowded mid February, especially around the first and fifteenth of the month as people start/finish a fortnight's holiday. Tolls on the highways. Any COPEC petrol/gas station will have maps, food, toilets etc.