Marvel at the size of Sao Sebastiao Cathedral. Go to a service if you can or simply take a seat and enjoy the peace and quiet.
You'll be spoiled for sightseeing choice in Ilhéus. Sao Sebastiao Cathedral is an ideal place to start here, but there are loads of other worthwhile places to visit. Milionarios Beach, North Beach, and Centro Cultural Bataclan are close to the principal church and and will cap off your day nicely.