That's been my dream for years. Have TWO ponds, one on top of the hilll, the other at the bottom. Creek with a flood gate in between for mini hydro dam!Flywheel storage is ok for short term power backup for small installations. Like a data center needing temporary power in a short duration outage while their diesel generators pick up the load. It just takes so much mass to create a lot of useable energy.
I'm a fan of pumped hydro. There is a large pumped hydro plant on the west coast of Michigan, they pump it up 400 ft or something, to a large man made lake. That's a lot of potential energy and easy to recoup. Of course the efficiency is always a lot less than 100%, so it's only pumped when excess electricity is on the grid for cheap, and sold when cost is high. It is a good balancing tool to reduce overall costs.
I also thought of each Windmill being a water tower to do the same as above.