Lots of folks have used micro-lending site Kiva as a way to earn miles and generate credit card spend to achieve signup bonuses.
The idea is that Kiva lets you fund an account by credit card, and Paypal waives the processing fees. You may a microloan, and once it is paid back you withdraw the funds. You generate spend and get your money back. Of course there’s some risk of non-payment, but the vast majority of loans repay (and since those loans are generally small, each exposure is small). You do need to be able to float the funds though for several credit card statements.
Now competing micro-lending site Zidisha is temporarily waiving credit card fees as well.
I really like doing well (miles) by doing good (making funds available for small projects in developing countries).