Slide Rock is an absolute must if you are in Sedona - it is a wonderful place to visit any time of the year.
Slide Rock is justifiably crowded during the summer; you can spend time in relaxation (I highly recommend bringing camping chairs!) or having fun riding the natural 'slides' in the river. I would suggest water shoes; be prepared to slip on the very slippery rocks!
The $20+ entry fee seems a bit excessive, but they do a remarkable job keeping this state park clean and the historic buildings well preserved. There are clean inside restrooms and a small cafe/convenience store open during daylight.