One of the best things about the Legend of Zelda series (or a lot of games in similar genres) is going around with recently acquired powers to open up new paths in old places and collect stuff. Unfortunately, this fun activity can easily be marred by the problem of Fake Rewards, which I mentioned in an older post. A good example of this is seen in Legend of Zelda: Ocarina of Time, where you often received large numbers of rupees as a reward, despite the fact that rupees were easy to acquire, there were not many things to spend them on, and all to often your wallet could not hold enough (which sent the rupees into the void). However, more than any previous entry in the Legend of Zelda series, Phantom Hourglass avoids this problem.
Most notably, Phantom Hourglass avoids Fake Rewards by having a wide variety of fun things to collect from treasure chests. Treasures are essentially rupees, but balancing the different ways they can be converted to rupees is a nice change of pace. Treasure maps are essentially just a delay on getting a real treasure, but the quest to salvage such treasure is fun in of itself. Ship Parts are fun to collect, and match the game well. Even just finding rupees is fine enough, since LInk's wallet is huge in this game and he has a lot of things to spend money on at various stores. But my favorite new form of treasure is the Spirit Gem.
Spirit Gems are a great addition. All combined, you need to collect 60 of them in order to unlock all of the Spirits powers. This is a lot, but you get them at a fast pace, so collecting them doesn't feel too slow, and the powerful rewards of equipping powered-up Spirits is a benefit worth the trouble. Having so many items to collect, with a powerful reward as the goal, is a great way to motivate and reward exploration.
As a whole, Phantom Hourglass is a big improvement in this regard over many of the classic games in the Legend of Zelda series. Too often in older games, the only real reward for exploration were Heart Pieces. Having to collect four items for each incremental increase to health somewhat cheapens each increase in health. Getting a health boost did not feel like a great improvement in of itself, but at the same time it does not feel like great reward for hard work. This is only made worse in Twilight Princess, where you need five Heart Pieces to get an increase in life. In Phantom Hourglass you just receive full Heart Containers, but only rarely. It turns it from being a generic reward into a rare but powerful reward, which suits a life increase better.
Rather than the old approach of treating all optional treasures and rewards the same, Phantom Hourglass separates rewards into different tiers of rarity and power, and creates a much more interesting game of puzzle-solving and exploration because of that change.