I can't believe this is my first post, but the topic hit very close to home, considering the fresh stitches in the back of my head
I just got back from Sedona and despite having done Slide Rock in previous years, I still got caught by surprise on those slippery rocks. Luckily there are a couple of ER/urgent care options in Sedona, including the one that stitched me up, which was only 2 miles from my TS in west Sedona. Thankfully, it was almost the end of the day for us and it didn't spoil the trip -- just a couple lost hours dealing with that injury. After hours of fun and sucessfully navigating my way around, I think I just got careless.
I'm not sure water shoes really help much -- most people go barefoot. Best advice is to forget appearances and just walk carefully and crouch a little, and beware that anything wet that you step on is going to be very slippery. The danger isn't really the natural water slide, it's in walking up and down the banks of it.
If you can't get there near opening time -- 8 am -- wait until mid-day, when the early arrivals have had enough and start to leave. Once the parking lot is full, they only let in another car when one leaves. We arrived just before noon on Monday and only had to wait 10 minutes or so to get in. Forget weekends, it's just too crowded.
Water quality was very good this week (rated "low risk") and they test it daily. Summer is really the only season for this, since the waters of Oak Creek are rather chilly and you'll be glad it's 90 degrees outside when you jump in.
In addition to the Grand Canyon, try to do a hike or two in the Sedona area. You may find that you prefer the experience of really being "in" a beautiful canyon rather than looking at one from a distance. In spite of the temperature, we were comfortable even hiking in the afternoon, with plenty of shade on the trails we took. A very popular and highly recommended trail is the West Fork of Oak Creek Canyon, just a couple miles up the road from Slide Rock. This is another one you want to get to early or else you will wait to get in, but grandmothers, little ones and everyone in between can all do this trail, and the scenery is breathtaking.
And definitely the Pink Jeep "Broken Arrow" trip is a must! There are lots of Jeep tours in Sedona, but the Broken Arrow trip is a truly unique experience and worth the price. Don't put it off until "next trip."
Enjoy your trip!