Ooh, ooh, I made the Mexican Rice for a mexican potluck at dh's workplace. It was eaten all gone, and people commented on how yummy it was. I made it with white rice, but I'm sure brown rice would be just as good - just make sure you cook it the correct length of time. It's nice, because it's a side dish, and not too many people bring those (at least at dh's work.)
The Polenta Mexican Casserole was good too. I made it for dinner tonight (the one Sandra posted) and it was good, especially with sour cream on top. Green onions would also be a good garnish.
Or, you could go really easy and bring a can of ff refried beans, plop it on a plate, and call it good.
Let us know what you decide.