Find an excellent driver for the treacherous road to Gangotri and only go in the dry season. Find a guide to take you to some of the local caves occupied by swamis. Meditate with them. Dunk your head in the Ganga for purification. Buy a small plastic bottle to take some of the river home. Wherever you stay it will be freezing cold at night and there is no hot water. Bring long underwear, hand and feet warmers and down jackets if you are there in October. Perhaps take the trek to the glacier at 11,000 feet by foot or donkey. Meditate as much as you can because you will have to take that same treacherous road back down the mountain!