A great hiking option, relatively flat, is the Deschutes River Trail, which runs right through Bend. The trail system is awesome! People do swim at McKay Park, but it’s not really safe or a swimming hole, necessarily. If you want that, I’d head up to Elk Lake. Especially in August, the water is warm and definitely safe – try the South Beach for a shallow hole. There is a lot of hiking up there, but it’s more mountainous. Have a wonderful trip!