The last time I was in Moab, our group rode Poison Spider, Porcupine Rim and the Slickrock trail, among others. Poison Spider and Porcupine can be done on rigid bikes, but it's easier with some suspension. With low enough gearing, I could have done Slickrock on my road bike. It's that easy, but still super fun. On the other hand, I agree with other posters that you will need to be prepared, especially in August. We were there in October and I think the temperature on the Slickrock trail was 30C. I went through almost 2L of water then, so I imagine you'll need 3 or 4 L for a trail like that in August.