Please note: This chapter isn't mandatory to finish the game.
Go back to the fortress, climb the big stairs to the right and speak with the woman in front of the door. Pay 10 rupees and get in.
Gerudo Training Ground
If you need some arrows, turn around and shoot at the eye, and if you need a fairy, stand where Navi turns green and play the Song of Storms.
Enter the right lion's jaw. Quickly get rid of the Dinolfos to earn a Small Key, then open the next door. Put on the Hover Boots, go on the platform on the left, then play the Song of Time so a block appears. Run to this block, then jump higher to get a Small Key. Look at the rupee engulfed in flames on the right, then jump on the platform on the left (the one with a switch on it). Step on this switch and quickly walk on the different platforms to pick up the rupee and quickly move to another platform. Pick up the other silver rupees, then open the door on the right from the entrance. Step on the red square, play the Song of Time so the blocks disappear, then put on the Zora Tunic and Iron Boots and jump down. Collect the rupees, most of the time by standing right under them, then by firing the Longshot at one of the targets. One of the rupees is at the top, take off your boots to swim towards it. Once you have them all, get out of the tank and pick up the Small Key. Get out of the room, shoot the Longshot at the target above the door on the right, and open it. Get rid of the enemies to get some arrows and unlock the door, then use the Megaton Hammer to hit the statues from their sides. There is a switch hidden under one of these statues, step on it to get a Small Key in a chest, and shoot an arrow at the eye behind another statue to get to the next door. Open this door, then shoot arrows at each of the four statues' eye, but be careful, you must do it in one run or else you'll have to start again. Next, fire your Longshot at the chest to pick up the Small Key. Get out using the Longshot and go back to the starting room.
If you want to, you can call another fairy, then get in the opposite lion's jaw. Fight off the two Stalfos, take the Small Key from the chest then open the next door. Pick up the five silver rupees, sometimes by using your Longshot, then open the next door (at the top facing the entrance door). Attack the Wolfos to get more arrows, then use the Lens of Truth and look above the mimic door. Shoot your Longshot at the target, then step on the switch and go straight ahead. At the end of the path, open the door, find another Small Key in a chest and come back to the Wolfos' room. Thanks to your Silver Gauntlets, push the huge block, then open the door. Destroy the three Leevers with your Longshot and sword, then open the three chests in the holes (one of them is invisible and contains a Small Key, the others contain rupees), but not the chest in the back which is booby trapped!
Go back to the first room and get in the middle lion's jaw. If you've found all the Training Ground's keys, you now have eight of them. Open the locked door on the left, use the Lens of Truth and look for a passage on the ceiling. Climb the wire fence, take the last Small Key, then go back down and open all the following doors up to the big chest containing the Ice Arrow. (You have two keys left - pretty useful if you've taken the wrong path in this room.)
Leave the Training Ground and go back to the Spirit Temple.