Material lighter than concrete for planters?
Concrete is a mix of four things: 


Water
Portland cement 
Sand
Gravel


The magic happens between the first two.  When the portland cement gets wet, it has a chemical reaction (which makes heat) that turns it from a powder into stone. 

The last two are just filler materials. It improves strength, reduces cost, and greatly reduces the heat of curing, since sand and gravel don't get hot. Without this, the heat would be unmanageable.  

Nowhere is it written you must use sand and gravel.  They make a variety of alternate filler materials where weight matters. First on my mind: crushed polystyrene. 

Obviously if you are replacing sand and gravel with something else, that means you can't use pre-mix concrete.  Pre-mix concrete is a bag with the portland cement, sand and gravel pre-mixed.  Since you don't want the sand and gravel, this won't do.  

For that matter, nowhere is it written you must use Portland cement.  For instance you can also use epoxy or a variety of other plastics (see TAP Plastics' website for quite the variety) -- either thick with a variety of fillers.  

Or you can work "thin", with fiberglass as a very thin but structural material. You can also do "thick" fiberglass, where you do a skin of fiberglass over a polystyrene block "core" - the styrene provides form and compressive strength, and the fiberglass provides tensile strength and a strong skin.  Fiberglass boats are built this way. 

No matter what you do, you'll need a form and a release agent to get the form to let go of the finished work.