I'm starting to see the value of getting a credit card for most hotel brands - spending counts as "activity" no matter how miniscule and that way your miles never expire. That's why I'm planning on getting one of the Hilton cards (the one without the fee) so that my Hilton points never expire. I also checked out Starwood's dining program, but there's about four qualifying restaurants in the entire city of Philadelphia, so unfortunately that option is probably out (I have their credit card at the moment, but that probably won't last forever).

Frontier Airlines - I have actually never heard of them (possibly for good reason - that sounds like a ridiculous requirement).