I'd say mechanical batteries, such as dams (i say 'such as', but i don't know of other examples as of now that are used as much as hydro, but i will add other hypothetical examples later). Pumped-storage hydropower is one real-world example of currently used low-tech solutions. "all you need" is a huge dam, a few of them, dispersed. they are often used currently and store tens of thousands of Megawattshours.
You could also try compressed air for a more exciting and risky storage. Gives a lot of opportunity such as 'oh no the high pressure tank is about to explode' (even though i'd build them in underground balls to maximise security. You know, balls being the perfect pressure container, and underground being perfect for not exploding everything all that badly.) Yust pump in air whenever you have too much, and let it out to power things when you need energy
Be creative, everything that stores any kind of energy is a battery (springs, and rubber bands, potential energy in form of bricks you lift up and let 'fall' down, connected to a rope which fires a transformer or just water in dams, kinetic energy in form of flywheels, internal energy in for of heat, even chemical energy, like for example converting water to hydrogen and oxygen, and reburning it, or even just batteries chemically as others might tell more about). Just make sure that the energy efficiency is high: Not much energy lost when given in, during storage and while getting out (so flywheels and heat, for example, are not optimal, even though i could imagine flywheels working not all that badly if you have many and constantly feed energy in or get it out according to demand)