We have visited Ubirr Art Site in the evening, just before the sunset. The rock art is interesting but quite washed out so unless i=you are a connoisseur or have a tour guide you might not be able to appreciate if fully. However the climb to the lookout was entirely rewarding - with a great view over Kakadu Plain and the sunset. Highly recommended. The park closes at sunset and there is a warden/park ranger ensuring that everyone leaves on time.