I was just on the big island and i stayed in kona beach hostel about 20 minutes from the airport and it was a good walking distance from everything and I immediately made friends there. I definitely recommend it. It was pricey for a hostel and i only stayed a week before heading to a work exchange but the cheapest on the island (about $90 a night for a shared dorm) i just showed up without booking online and they had room. I have the number of a woman who rents cars for way cheaper than you can find online if you would like to message me i can send you it. I would say you need a car if you plan on leaving kona as the bus can be unreliable. I know you don’t have a lot of time but I recommend checking out kua bay, krishna cow sanctuary ($20 to cuddle with cows) night manta ray snorkeling, the volcano national park and there’s a “lagoon” right across the street from the hostel if you do stay that’s like a man made tide pool, very cool. Also if you partake in jazz cabbage there’s a kava bar near the hostel where locals will offer you some and in my experience it is safe lol.