Do a Great Escapade fare, we did and it was fantastic. Virgin Atlantic, Singapore Airlines and Air New Zealand allow you to fly on any of their routes up to i think 12,000 miles (you can buy more miles in groups of about 5,000 i think) for one set fee and then you just pay taxes for whatever airports you want to stop at. We changed the dates of our travel loads of times too and it was all free. Excellent. i'd go again no question whatsoever.
http://www.thegreatescapade.com/
Have fun and hope that helps someone.