Hawaii's Kohala coast contains some of the world's most luxurious and beautiful resorts. The good thing for you is that you don't have to stay there to benefit from them! Hawaii's laws guarantee beach access for everyone, so you can often park at the hotels in designated areas, and enjoy a stroll along the hotel's lush grounds.

At Mauna Kea hotel you can park near the left side of the beach, and dive in for a fantastic swim. The beach is a long crescent of lovely yellow sand. There are often small waves for kids to bodyboard on. The bouncy waves make for a relaxing swim. Aaah, perfection! The right (north) side of the beach, or the area out near the reef is not recommended as it can be rough, though some people do snorkel there.
At night, there couldn't be a more romantic spot. A short path with railing looks out over the golden curve of beach and the pale green water. Stop along the path for a kiss under the moonlight. Just near the path is a lookout with a large floodlight. The floodlight accentuates the gorgeous colors of the water, and manta rays are attracted to the light. Most nights you can watch manta rays glide serenely by just a few feet below. It's a great view!
A path leads from the right side of the beach up the cliff and beside the ocean, past gorgeous views! Make sure you walk the Ala Kahakai Coastline Trail. 
The Mauna Kea has an exceptional collection of ancient eastern art and sculpture scattered throughout the hotel. The friendly concierge at the desk will give you a brochure detailing the art if you ask about it.
In the daytime, it's wonderful to stand in the open-air lobby, feel the gorgeous breeze, and look out on the incredible colors of the water. Paradise! Nothing could be nicer!
If you're hungry have a meal at Hau Tree Cafe, right by the sand. You can order a tuna sandwich or burger for about $16. If you choose this option, you can use the valet parking and get your ticket validated at the cafe, which is handy since the beach parking is often full.
If the beach parking is full (which it usually is after 8 or 9am), check out these beaches instead:
Waialae Beach (69 Beach) is just glorious on calm, windstill summer days, and has some great snorkeling. Big shade trees stretch over the sand making this a nice place to take kids without the risk of sunburn. On winter days the water can be more choppy. Also, you have to choose where you swim since some areas have mostly sand and others have lots of rocks. 
A beach with stronger surf and not as much shade, but plenty of parking is Hapuna Beach. The water is a gorgeous turquoise and it is lovely to jump in it. There is some shade by the rocks on the south end in the morning.
After, have a meal and do some shopping at Queens' Marketplace and Kings' Shops.

Directions

Mauna Kea Hotel is located at 62-100 Mauna Kea Beach Drive, Waimea HI 96743, call (808) 882-7222. 
Thirty miles north of Kailua-Kona, a 46 minute drive. Take Hwy 19 and turn onto Mauna Kea Beach Drive near the 68 mile marker.
There are only 30 parking spots for the public so arrive early for a swim, or else use the valet parking and eat at Hau Tree Cafe, getting your ticket validated there.

Click on map for interactive view