While any shirt could probably do the trick, you could also spice things up with a western-style shirt. A pop of color from a plaid shirt can really draw attention when the rest of your clothes are in neutral colors. Another casual option would be a henley, a perfect casual wardrobe staple.
Hurley V-Neck T-shirt
Legendary Whitetails Fleece-Lined Shirt
Original Penguin Henley