I've been using my cash back reward credit card for everything I purchase i.e. gas, groceries, food, shopping etc and of course paying off the balance in full each month. In 2013 I charged $27k worth of stuff to it I got $646 back in rewards or 2.3% on the money that I spent anyway and it's not taxable because it's considered a discount
Also, when I shop online through FatWallet I get additional cash back from them too. Though the discounts and items offered through Fatwallet can be selective I did save 6% on camping stuff I bought at Campmor through Fat Wallet.