There are 3 bomb bags to obtain and 1 capacity upgrade that affects all 3 bags.
Go to the fishing hole, and find the sign that says: No Littering! You can get a bottle if you fish on the right side of the bridge that is near that sign. Its darker water than normal.
Unlike other Zelda games (where you got attacked if you hit the Cuccos), in Twilight Princess if you hit a Cucco about 8 times, you'll be able to control it for a short period of time.
L-target and perform four sword thrusts by tilting up on the control stick and pressing B. Make sure to leave enough room so only the fourth thrust connects. The Darknut will then swing its sword twice; immediately after the second swing, perform a Jump Slash on the Darknut. This will cause the Darknut to raise its sword in the air, leaving you free to attack it.
In the final boss battle, cast the fishing rod to distract him and get in a few easy hits.
Donate 1000 rupees to Goron inside the Malo Mart in Kakariko Village (you must progress past the third dungeon beforehand). Talk to the old Goron outside the shop and complete the quest he offers. Once completed, the second donation amount will be reduced from 2000 to 200 rupees.
To easily find the locations of all Heart Pieces, go to Fanadi's Palace in Hyrule Castle Town and pay her 10 Rupees for a Love fortune. You will then see an area in the game that will contain a Heart Piece around it, obtainable in some fashion. You can keep paying her to show you other locations, but she will sometimes repeat the same location a few times over.
After locating the Gerudo Mesa and Cave of Ordeals in the Gerudo desert, you are able to unlock fairies and later Great Fairies at each spring around Hyrule. Smaller fairies can be bottled, restore 8 hearts when used, or when you run out of hearts. Great Fairies will give you a bottle full of Great Fairy Tears, which heal you full and temporarily boost your attack, provided you have an empty bottle and no Tears in your inventory already.
Both have an infinite supply once unlocked.
After donating 1000 rupees and completing the hot springwater sidequest, donate 200 more rupees to Malo Mart. A new Malo Mart will open in Castle Town and magic armor will be available for purchase there. The amount owed can be reduced (see Secrets section below).
You can unlock certain rewards for returning Poes back to Jiovani.