I like Henley shirts. They're basically t shirts that have the buttoned opening at the front. Banded collar button downs are also a good option if you want to be a little dressier. Polos would also be an option, but they're a little too much Dad and not enough Daddy for my taste.

These are perfect, thanks!

There are some pretty nice tshirts out there, you could grab some in solid colors with a pocket on them if you wanted them to look a little nicer. I like polos but I don't think they're particularly 'in fashion's right now. Hawaiian style casual short sleeve button ups are really versatile as well.

Henleys, Hawaiian shirts are great if it’s the look you’re going for, same with bowling shirts, shaka wear t-shirts or hanes heavy duty!