Z-Fold Card

 The easiest fun fold I know of is what I call a 'Z fold' card.  You make a standard card and add one extra score to the card front.  I am in the US so all my card are A2 cards--sorry to my Stampin' friends in other countries.

Starting with an 8-1/2" x 11" piece of card stock you have 2 choices:

1- Cut in half to 8-1/2" X 5-1/2".

2- Score on 8-1/2" side in half at 4-1/4" and half again at 2-1/8"

Or you can cut and score your 8-1/2" X 11" card stock in the other direction.
1- Cut in half to 11" X 4-1/4"
2- Score on 11" side in half at 5-1/2" and half again at 2-3/4"

Fold into a Z-Fold card with the top flap folded back on itself.  From here you have many options. Here are a few ideas to get you started. 

A die cut element that extends over the folded edge adds interest to your card.

Instead of a punch or die cut, you could add smaller layers to the folded edge.

You could also adhere a full layer to the folded edge, so it looks like a standard card from the front.  (Just be sure to only add adhesive on one side of this layer!)

Inside the card above.