Power Berries
Power Berries increase your stamina level. You can view which ones you've gotten by viewing one of the two papers on the wall in your kitchen (The other one's recipes)
Plow your field with an un-upgraded hoe to eventually get one.
Buy one from the Salesman at the Flower Festival on the 24th(?) of Spring.
Win the Egg Festival
Become friends with Basil
Fish in the Ocean off the Dock
Go to the Goddess Pond and throw in some of your produce at Full Stamina (when you haven't done any work). Wish for Strength.
Smash the Boulder on the frozen pond with a Golden Hammer.
