Lake Mead
People come to Lake Mead primarily for boating and fishing. Adjacent marinas offer watercraft rentals, restaurants, and paddle-wheeler cruises; the turn-off for them is just past the entry gate. A few cultivated areas allow for swimming but they are not designated swim beaches, so no lifeguards are on duty. In fact, the National Park Service highly recommends wearing life jackets, as high winds come up fast on the lake making for potentially dangerous swimming conditions. The rocky Boulder Beach swimming area is about 2 miles past the visitor center.
A fishing license is required within the states of Nevada and Arizona, so if you plan on fishing Lake Mead, get one.