The obvious (but often incorrect) thought is gravitational potential given mountains.
What are the challenges to incline rail lifting a lot of material up a mountain to roll down later?
Water freezes, slopes with snow can be unstable, cold weather is hard on gears, etc.
Large (in old mine shafts and chambers?) thermal mass storage has potential, not dissimilar to "heavy" (not suitable for EV) recent battery technologies.
Are there any serious tidal races in Scandy lands that can be tapped?
If you're talking about seasonal storage I would have thought the natural fit would be power-to-gas or power-to-liquid syn-fuels? Absorb all those extra kWh from the sun in summer, store it for 4-8 months and then burn it during the cold, dark winter? Energy storage density higher than gravitational potential energy and less energy leakage than thermal-mass storage due to non-perfect thermal insulators etc?
FWiW I talk about storage liquid gas hydrogen byproducts a lot - I'm in Western Australia, we're putting up small country sized PV farms to power billion tonne per annum mining operations and with no actual "mountains" (although many mesa's) we look to ammonia | methanol and "infinity trains" rather than to hydro dams or gravity stacks, etc.
The GP comment mentioning "Scandanavia" seemed like a good excuse to spitball ideas with mountains - but as outlined they're no easy fix either - you can't just put a big dam anywhere without potential dangers and cold climates have issues with freezing, expansion, contraction, etc.
Where I'm at we're more concerned with overnight power for massive 24/7 continuous operation than with longer nights and less light in winter .. that's simply not a thing here.
When I said Scando I meant Finland. So, mountains are out of the question! Also tidal races.
OTOH there's mines, but oddly enough I've seen no discussion in the media of using them.
So.. there's no simple land/geographic features worth exploiting. OTOH some storable liquid/gas is more realistic. There's also sand batteries - Helsinki city has one - but they're maybe not so easy to do residentially/DIY, I would expect the neighbors to complain.
