I have a hat obsession and so my favorite moment from my most recent Vegas trip might just be Chapel Hats.
Located in the Venetian, this is a must on your list! They have the cutest hats - from faux furry to vintage-inspired. My guy also had fun and bought one page boy hat - I bought three, well two and one headband with a veil and feathers on it.
I wish the store was a little bit bigger because it can get cramped with so many people in the store but they have plenty of styles to choose from. The girls that work there are also great - super friendly and helpful.