Question
What is the best way to use Thank You points into CASH?
Answer
Let me say that while I'm not exactly on the same page as yourself, the Mrs. and I have been very happy over the last couple of years in redeeming our TY points for gas cards, which anyone will attest probably comes pretty darn close to what you have in mind, given that gas, for most people, is an unavoidable and recurring expense. Anyway, that's what we do.
Answer
Gas cards are probably one of the quickest routes to cash, but IMO not the best route as for Gas I can still get a 5% rebate on my Chase Rewards Plus (at least for the few of us who still have the cards).
Other friends have the Citi Driver's Edge for 6% for a year, as well. And, many other cards like Chase Freedom still get you somewhere North of 3% for gas.
I'm partial to Marriott or Hyatt G.C.s for the double dipping opportunity of still getting the program points for the stay. (Of course there are lost bonus M.R. points if you have the M.R. Visa, but since the only Hotel CC I use anymore is Amex SPG, that point is moot for me.) But, that is only important anyway if you are planning a stay.
Of course you could also buy the certificates (like Marriott) and gasp, sell on eBay, where Marriott Certs often get something like 95% of face value if you have a good seller rating. It is against the conditions of the certificate, but the morality/ethics issues are up to you.