I'm off to Santiago for 3 days - July 4-7. What is there to do outside of Santiago? Any interesting day trips? Anything in Santiago that's of interest and not on any tourist maps?

I'd definitely recommend going to Valparaiso for a day - you can rent a car and drive (Preferably from SCL so you don't have to drive in the City) - or take a bus service - they are clean modern safe and cheap. Valpo is a charming place - but as your weather may be "iffy" just plan "the clear day" will be the day I go. Weather in central Chile will be much like California. And you are there in the winter. So you will have wet days and beautiful days. Viñas are not far from the City, either.