The absolute best part of it (in my opinion) is reading the books along with the website. Right now only the first book is open on Pottermore, but you click it and go through parts of each chapter - there are little things to collect and discover along the way and Rowling has added many back stories and bits about characters and places for us to read - it's really awesome!
There are also other things to do like potion making and dueling (I haven't experimented too much with these yet).
You just have to explore!
