MapleStory is a free to play MMORPG with an in-game cash shop for cosmetic items. It’s a very addictive 2D platformer style game which I’m guilty of putting many hours into. Despite its appearance, it is surprisingly fun for all ages and has a good welcoming community for you to join.
Like many in this genre, MapleStory involves the RPG elements such as defeating monsters and completing quests for experience points which level your character up. However I think what separates this from the many others is how simple it was to get the hang of, with a very gentle learning curve and helpful interface. This was important to me when I started playing as a young gamer around 2008 as it meant I easily got attached and have since returned many times.