In the "Spaceship Earth" section you can see a method of moving a planet using the gravitational pull of another body. Variations of this technique abound. By using techniques such as these we could bring together planetary bodies such as Mars and Europa to produce a new planet with approximately the mass of the Earth and with plenty of water. By then seeding this "New Earth" with the appropriate flora we could produce an atmosphere with enough oxygen to allow animal life. We could then seed the planet with the appropriate species to make it a livable planet for humans.
Admittedly this project would take thousands of years, but given the risk of losing all humanity due to a global catastrophy, either manmade or natural, this seems to be a worthwhile project.
A much speedier method might be to deflect comets onto Mars. This would add water in smaller amounts and possibly be habitable in a few decades.