Just been there and I agree with the above. It was fairly hot in March so it'll be scorching later.
Slickrock Trail is ok but I wouldn't call it fun. More of a struggle and not a great deal of flow as it's VERY up and down.
Porcupine Rim has to be the ONE trail. The most fun, stunning and truely awe filled ride I've done. Two miles of up at the start then 12 mile of down rocky, technical, fast and flowy sections. The last bit is very tricky and was advised to hike the worst bits as there is a terminal drop to the right.
There are regular shuttles from the town (Poison Spider Bicycles is one) at around $20. The ride back to town isn't that far either, some on road but a lot of cycle path. Actually quite a nice warm down ride if you ignore the huge vans that they call cars over there whizzing past.