Fun free virtual worlds for tweens?

Runescape is a really good game, its really easy to play except at the very beginning but every new play finds it hard to play at first. You can either train your character to make them stronger, or gather up loads of money to but really expensive items. (expensive is like millions if you have $200,000 that's considered chump change). Or if you don't want to do that you can stand around Lumbridge and talk to dozens of players for hours on end. I will say if you try to train or make money it is extremely time consuming when you first start playing. They have a Trial version of the game but its not that good at all. So you can just start out for free right off the bat, but you'll have to do a quick tortiral to get a basic understanding of the game.