In terms of 'cash-back' if you are chase member you can book your flight on travelocity or hotwire using the Shopping Portal for an additional 2pts/$.
My chase strategy combined with my Chase Sapphire (2pts/$ on travel) yields a total of 4 Ultimate Rewards points / $. On a $6K purchase you're looking at 24K Ultimate Rewards points which is almost enough for a RT USA flight on United or 3 British Airways Short-Haul flights. Not to mention you still earn regular frequent flier miles from the flight itself.
I believe Discover also has cash back for these sites as well if that's your thing.
EDIT: I should clarify before anyone yells, you earn your FF miles when booking flights-only on these sites. Hotels or Package Deals of flight-hotel-car combos you do not earn any FF miles.
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