The use of "hempcrete" is one of the greenest things you can do in home construction. Self insulating and lighter than concrete with a zero carbon footprint. Other big items are recycled glass counters, LED lighting, and on demand water heaters
One of the least expensive ways to create a green home is to build a tight building. Things like enclosing insulation on all six sides, foaming behind and around electrical boxes on exterior walls and ceilings, caulking wall base plates to floors, caulking gaps between framing members on exterior walls, foaming holes in top plates at attic...etc.
These things take time but the material costs almost nothing. Build your own home and make it happen!