A collectible that you can acquire on this site is chocolate frog cards. More than just a treat, Chocolate Frogs allow you to collect famous witches and wizards cards! Once purchased, you can go to "Chocolate Frog Cards" in the menu at the top right (where your profile is and where you found the encyclopedia for example) to open them!

Chocolate Frogs can be purchased at Zonko's Joke Shop, in the upper left portion of the forum area there is a button that allows you to enter the shop.  This shop is located in Hogsmede which is an RP area that is not available to students until the third year. However there is a way to get around this restriction, that being the use of a fake permission slip item which can be found in Weasley's Wizard Wheezes shop in Diagon Alley. This will give you access you this forum area before your third year. Remember that when you are roleplaying, it is possible that you might be in character discovered by a Prefect or a Professor and you -might-  suffer detention as a result but mischief such as this is what makes being so young so much fun!

The cards are random, so you will not know what cards you have recieved until you open them up. Because some players might enjoy experiencing the thrill of discovering what faces are contained in these cards, please do not state any card by name if you wish to trade cards but rather reference them as Bronze #18 or Gold #3 if you choose to discuss them. [Link]

How about Trading?

Yes, you can trade Chocolate Frog Cards with your friends. We have two places for this, the first being Discord. We have a special channel dedicated to card trading entitled #card_trading. We also have a thread on site located in The Leaky Cauldron in the At the Counter forum area in a thread entitled Chocolate Frog Trade and Shop [OOC].