A tote bag is perfect for office and off-duty days. A tote's spacious interior will easily house your essentials plus a spare pair of flats.
if you're in a thrifty mood, your best bet is this Dooney & Bourke Tote Bag. It's a great piece, while not very expensive. Another strong contender for your wardrobe is this Kensie Tote Bag. While the price of this piece is not too high, it will surely attract a lot of attention. You may also want to look at this Calvin Klein Tote Bag. This item is not expensive either, but it will make a difference in your wardrobe.