If you can afford to spend a little more, look at this DKNY Bucket Bag. While the price is very reasonable, this item is an easy wardrobe update. It's also worth looking at this DKNY Backpack. This piece is extremely popular and, just as importantly, not too pricey.
If you're not a thrifty type, then what you're looking for is this DKNY Rucksack. It's more expensive but worth your money.