The swimming beach at Colter Bay on Jackson Lake in Grand Teton National Park is a great place to cool off on a hot summer day while taking in the views of the Grand Tetons to the west. Cold water runs into the lake from the many canyons and valleys full of glacial melt on its western shores. From this area you can see the northern canyons, which are largely inaccessible to foot traffic and do not have many trails cut into them. The sights from the beach are quite amazing, and while the days don't usually get very hot in the Tetons, you will appreciate the chance to get in the water after a day of hiking.
Make sure to bring some footwear you can get wet; the rocks on the beach are large and square. There are kayak and boat rentals at the nearby marina, and you can pitch a tent at the Colter Bay Campground. The Colter Bay Lakeshore Trail and the Hermitage Point Trail are both nearby, and there are two small islands not too far off the shore that are great destinations for a kayak or stand-up paddleboard trip.