+1 to the SallieMae suggestion. There's a monthly $250 cap to the 5% back on groceries, $250 on gas, and $750 on bookstores (Amazon counts as a bookstore). We do most of our shopping -- groceries, household items, gardening, etc -- at Fred Meyer, which all counts as a grocery store, so we hit $250 pretty easily on one card. Which is why my husband and I both have a card, so when we hit the cap on one, we switch to the other.
We also have the Discover It, which I mainly signed up for because I was planning to buy a Macbook and the card gave 5% back at the Apple Store. lol.