If painting isn't your thing...it isn't mine......
You can find non-straw cowboy hats on sale at Cavenders for less than $40 most of the time. I've decorated two -one black, and one pink. I have a red hat sitting in a closet that needs some decoration before next season.
You can buy red or blue beads and put them around the rim as in a band. I've taken an old lanyard and used the ribbon part to put around the rim.
I took an old cap that had seen better days and cut the logo off of it, sewed around it to keep it from fraying, and hot-glued it to the front of the cowboy hat. I've also stuck an "I'm A Texan" pin on the front of a hat.
I just wandered around Academy, FanAttic, and my house looking for little things I thought I could put on a hat. Sortof surprising what you can find and stick on a hat with a little glue gun.