What I find really useful - and of course very easy to do - is to darken our home on a hot summer day with curtains while being at work. Doesn't cost anything and helps us to cool our rooms to a very comfortable temperature.
In Spain, summertime is plenty of solar energy. Solar water heating for shower (http://www.duchassolares.es/ or more homemade ideas as n interesting solutionhttp://www.motherearthnews.com/Renewable-Energy/1979-09-01/A-Homemade-Solar-Water-Heater.aspx) or for cooking () ... Interesting also to be done with children