What is your thinking on not wanting to open credit card accounts? I can't think of any negatives, and we have used the free miles for a couple flights this year. The Mad Fientist wrote a great post about his planned trip pretty much around the whole world for free
http://www.madfientist.com/american-explorer-award/I think a free trip is easily worth the 30 seconds it takes to apply for a credit card. We use our card just like cash... charge something and then pay it off that month.
Not necessarily a hack, but traveling slowly is fairly cheap. We travel full-time and it costs us less to live abroad while living quite luxuriously than our somewhat frugal cost of living back in the US. We prefer to not fly, but being flexible on days makes a huge difference
We recently booked a flight from Mexico to Taiwan, and flights were $1200 - $1800 each on peak days with no great mileage options, but if we waited a few weeks and flew on a Tuesday we flew for free with very few miles (which again we got for free from a credit card)
Somebody mentioned couch surfing, but there are also options for house sitting that many people use. You basically agree to watch somebody's house and take care of their plants and pets and they let you stay in their house for free...