Zion Cycles is there in Springdale. It's a smaller shop but they have rentals. You'll need to backtrack towards Virgin to get some quality riding in. If it's not too hot out, you can ride the JEM Trail from the bottom trailhead in Virgin up to the top and back which is just under 15 miles. If is IS hot, maybe go ride the trails up on Wire Mesa. The mesa trails are *usually* cooler. We ride Wire Mesa clockwise and then ride it back the other direction to get 12 miles total.
If you want to drive all the way back to the St George area, Church Rocks is a great intermediate option.
Over the Edge bike shop in Hurricane has by far the best rental bike and parts selection in SW Utah if you care to drive there from Springdale.