I was there in May last year. IT was in the high 90's then. So, in summer it will be same or higher. But very dry heat.
Hoover dam is about an hour away driving. The Grand Canyon is more like 3 hours or maybe more, driving, each way. No beaches there, you are on the rim.
The hotels ALL have pools, some more magnificent than others but all are lovely.
There are lots of things to see for free, too. Most of the hotels/casinos have some kind of display or show that is available for tourists on a regular schedule, like every half hour on the hour. You have to read about them and plan to be there. Or, they have areas that are open and you just walk through.
Plan on doing a LOT of walking. And outside, too, in the heat. But, it is really doable because you go soon into perfectly airconditioned places to cool off.
I found people there to be suprisingly happy and friendly, not jaded or sour as I would have expected for such a tourist town. Most of them said they really liked living there and it's a good place to bring up a family (not the Strip, though)
If you have only a short time, I would just stay around the hotel, skip Hoover dam and do stuff on the Strip or in the old "downtown".
I wanted to see the Neon Museum, but you have to get reservations well in advacnce.
The drive through the Red Rock area was really nice and only a half hour from the STrip. There was one hike (only about 3/4 mile each way) that went up to a tiny but refresshing waterfall. So, gorgeous and so different from my verdant home.