you can definitely get away with just tee shirts etc. Pack layers as the temps change a lot through out the day. Some people did wear polo shirts etc to dinner but it wasn't necessary. most people did change for dinner but that was mostly just to get out of the clothes you went fishing or horseback riding in. Nothing fancy was necessary.
One thing I would recommend is hand lotion, chapstick etc. the air at Brooks lake was very dry. you can also request a humidifier at the front desk if you need it.
We had such a great trip. you will love it!