Anytime can be interesting depending on you. It is warm to hot in the summer with monsoons in the later half. Half our annual visitation comes in the summer. It can be cold and even snowy in the winter, but it is very quiet and you may have the park to yourself.
Spring can be pretty, but the weather is windy and changeable.
Well, people come here to see the petrified wood and the Painted Desert. Two of my favorite areas are Long Logs and Blue Mesa for just those reasons.