Free cooling benefits depends onIt would be good to have a more detailed description of your climate zone and building use. Humid areas may benefit less fromHVAC equipment for a better answer in what would be free cooling because of the extra humidity added to the indoor airfor you.
If you are asking abouthave a housesystem that mixes return air with outside air for cooling and ventilation, free cooling is easily availablecould be shutting off the cooling equipment while keeping the fans on. This could be done by simply opening windows as well. The excessive humidity (and the energy included in it must be considered).
As you have a stream near your house, a chilling beam could be a possible solution. The stream water would used in its natural state, or chilled before flowing into the chilling beams. The advantage of using chilling beams is that the heat lost though radiation is felt differently from the air temperature, and a slightly higher temperature would be considered comfortable. 28 degrees Celsius instead of 26 degrees Celsius (this number depends on your clothing and activity level though).