The weapon exchange coupons are the most note-worthy of all the rewards because they can currently only be obtained through Coco Leaf rewards. The coupon will exchange all 3 of your weapons to weapons for a different class. This is great for players who want to reroll to another class and because of their rarity, you should only really use them on PEN weapons to swap to other PEN weapons. The coupons have no expiry date so you can keep them forever if you want to just in case you ever want to swap to another class.
However, if you are a new player you probably don’t care too much about the coupons and will not be using them for a very long time, if ever. If you don’t want to get the coupons, then the Wizard Gosphy pet is a very nice choice. The wizard gosphy is a pet which can loot items for you automatically. It’s recommended to get 5 pets as early as possible to make things a lot more convienient for you when grinding. You can buy pets with real money or do the quests to get free ones and buy more from the central marketplace with silver.
The wizard gosphy is also a good pet to have for other reasons. Basically pets can also “breed” to create a higher tier pet with more skills and higher loot speed. However, pets have different categories and can only breed with other pets in the same category. Wizard gosphy is the only pet that can breed with all pets.