You have mentioned water parks a couple times. Perhaps get out of Vegas a day earlier and stay a night/day at the Great Wolf Lodge in Manteca, CA? Your girls are the perfect age for Great Wolf Lodge.
On my last trip to Vegas we made an entire afternoon out of visiting the chocolate stores (Hershey’s World, M&Ms, Ethel Ms). My kids were 11 and 4 at the time - they had a blast.
Zion or Yosemite - both are amazing. Both will also be very busy in summer. Have they been to SFO - perhaps a day there to see the sights?
Not sure if they would be in to it - a day (or at least part of a day) on our next trip will be
pressed penny collecting around the strip. We have done this a few times at Disney parks and it has been a hit.
Vegas also has a
minor league baseball team - might be an interesting cultural experience. Maybe you will be in town for free trucker hat night (July 15)?