Mauna Kea Beach is consistently regarded as one of the Big Island's top beaches, and it's hard to argue otherwise. Fine white sand, turquoise waters, fun but not abusive surf conditions, restrooms and showers, shade opportunities beneath the trees...the list goes on and on. About the only thing that might keep this from living at the top of your list of dream beaches would be the undeniable presence of the resort; if the resort culture makes you uncomfortable, find another beach or stick to the south end, where the public and locals have a great time.
This is a great beach for families as the restrooms and showers are close, the sand is fine, and the surf conditions are generally friendly. Always be careful, however; the surf can and does rough people up from time to time. Lifeguards are on duty at the north end for guests, so if you are swimming at the south end you are on your own. Bring a boogie board to surf close to shore or a paddleboard or surfboard to surf the wave that breaks off of the north point.
As popular as this beach is, you can probably guess that parking is an issue. Visitors access the beach through the Mauna Kea Beach Hotel entrance gate, and a limited number (30) of free passes are allocated every day. Arrive early for the best chances of getting one, or aim to show up late and take advantage of some turnover. From the parking area it is a five-minute walk downhill to the beach. If you show up and can't get a pass for Mauna Kea Beach, ask if you can get one for nearby Mau'umae Beach a little farther north. If both are out, don't give up; head back south for the equally impressive Hāpuna Beach.
A quick warning: While surf conditions are usually friendly and fun at Mauna Kea, the ocean always has a right to change those circumstances. Waves here can and do lift people up and slam them down again with great force when conditions are whipped up. It can be tough to respect rough surf conditions when you are on vacation and have a limited time to spend in the water, but staying out is much better than getting injured. Remember, there are no lifeguards here to bail you out of a bad choice.