Hana Hou Cafe is the Southernmost Restaurant in the entire United States. That alone made a good enough reason for us to try it! We always look for the unusual. It's family run, home-made cooking, friendly, and delicious. We'd easily eat there again, and highly recommend it if you're heading south on the Big Island!
To find it, look towards the ocean for Spur Road (right next to a small shopping center) in Na'alehu. You should see a sign, and can turn into the shopping center to access the restaurant if you miss the road (as we did).
Park, and go right in. The "setting" is a basic diner. Many locals eat here - some tourists. We were the only tourists we saw, but their guest book had others listed from points afar. It wasn't busy when we stopped, but we were off typical lunch time, so I'm unsure of peak hours. They have plate specials of the day (which we got) and menu ordering too - quite a decent variety on their menu actually. For Hawaii, this place is not expensive at all.
For something different, consider trying their Lilikoi Lemonade. Four of the five us of liked it (and the 5th is my youngest, pickiest eater who doesn't care for regular lemonade either - he did really like the food). Their desserts looked awesome too, but they gave such huge portions that we were stuffed at the end of our meal - and on our way to Kailua-Kona, so weren't sure about keeping desserts fresh in the van while we explored South Point, etc. Another time we'll plan accordingly and give the desserts a try though!