Hands down lucky board is the best place to spend coupons for power, spend them on med refine runes.

The number of freebies you get from the board cannot be quantified but it's roughly 80% returns on investment imo.

When you go 1 round around the board you get a lucky ninja pack, and the level 4-5 magatamas you can get from them can really boost your power. Not to mention you can get lucky and pull the super rare ninja which is Hashirama this week .