Houston has two airports. Intercontinental is much larger and generally has more/cheaper flights, but Southwest only operates out of Hobby. Neither of them are particularly close to the venues, so it's really up to you. Intercontinental is on the north side of town, Hobby is on the south.
Wear something that makes you feel good! "Casual dressy" could describe it, nice, but not black-tie. Houston in September is still pretty warm, tho, so leave your wool suit at home.