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?
It's a poser alright.