If you're not planning on spending much, consider this FjallRaven Rucksack. It's a great piece for a good price. Another strong contender for your wardrobe is this Mandarina Duck Backpack. While the price of this product is not too high, it will certainly attract a lot of attention. You might also want to consider this Fjäll Räven Backpack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
If you're after a bargain, you're in luck with this MCM Rucksack in stock and waiting for you. You most certainly will love this piece. It's also worth looking at this Kenzo Rucksack. This piece is extremely popular and, just as importantly, not too pricey.
If you can afford to splurge, then you won't be disappointed if you check out this MCM Backpack. It's an expensive but smart option.