Okay, I have nothing to do right now.
Let's just hand wave everything. As part of their breakfast for the big flight the crew of the Enola Gay and all other planes involved drank an immense amount of lead paint as Japanese saboteurs modified their equipment to point them towards Hawaii. Meanwhile, another team of Japanese saboteurs used sleep hypnosis to convince them that any attempt or clue that they were heading for Hawaii was enemy interference and could be safely ignored. Also, a Japanese scientist managed to figure out the formula to a chemical additive that would make fuel last longer, but unfortunately he, his team, and the only known copy of the formula was destroyed in a bombing raid the day before. Also, any saboteur that knew about it died somehow. Oh, one final one: air defenses in Hawaii are run by imbeciles because the Pacific campaign was so far away at that point, and the US thought they didn't need good men there.
Right, that should be everything.
We have one tricky issue in that Honolulu itself wasn't a military powerhouse the way other US cities were during the war, that probably would have been Pearl itself (feel to correct me). To say nothing of the fact that that's where I'd lob my magical ball of death if I were the Japanese High Command. But since the OP is asking about an attack on Honolulu, let's assume the Japanese targeted the city for terror bombing as revenge for all the raids on the Home Islands.
And so, on August 6, 1945, the Enola Gay drops Little Boy on Honolulu, with ground zero as Honolulu Hale (the seat of city and county government).
Nukemap doesn't really help us, as it's geared towards modern Honolulu (which, ironically, is the population size of 1945 Hiroshima). But let's say it's the same effect as OTL Hiroshima. The core of the city is basically flattered, with tens of thousands of casualties and many more to come from radiation, starvation, disease, etc.
Also, the minute the bomb detonates the handwaves end. The hypnosis wears off, the equipment self corrects, the chemical burns off, the day shift comes on at air defense, etc.
The crew of the Enola Gay are horrified as they realize they are in fact over Hawaii, and Honolulu has just evaporated. They may have a chance to ask what the hell happened and what they've done, but it won't be very long as their fuel is exhausted and the plane crashes. Alternately, they are taken out by the air defenses around Honolulu or Pearl (both out of range of the atomic blast). Either way, they don't make it through the raid.
In Washington, a horrified Harry Truman is informed that Little Boy has detonated over Honolulu and the city is gone. Rescue efforts are ongoing, but it's a lot like Chernobyl in OTL - these poor bastards have no idea what they're running into and will suffer thousands of casualties in the weeks, months and years to come. Of course, it's impossible to hide the destruction of Honolulu from the American public, and panic is triggered in many US cities - especially those on the West Coast as rumors speak of some Japanese superweapon. I also wouldn't be surprised if Japanese-American civilians have a tough time of it.
Truman was famous as the man who said the buck stopped with him. It's certainly an interesting thought as to what he would do with this mess. On the one hand, I could see him taking responsibility and becoming the first president to resign (though that probably won't save him from investigation). On the other, he might try to say that it was a Japanese strike but I don't see what benefit that would have. Either way, Harry's screwed.
As for the war writ large, Boxcar drops Fat Man under very, very, VERY tight security on August 9, probably as OTL on Nagasaki but also possibly cutting Hiroshima's reprieve short - no idea. I'd actually imagine that the war ends mostly as it did OTL since the Soviets entering the war was just as, if not more of, a cause of Japanese surrender as the atomic bombs.
The Soviets... ah, internationally, this has some interesting possibilities. I see Stalin still wanting the bomb but also puffing on his pipe with the thought of "what idiots." The fiasco might make Stalin less cautious than OTL, but he still has to deal with his own postwar problems and weaknesses. The defeated Germans remain defeated, but probably are a bit resentful - these are the fools that helped end the Thousand Year Reich? I'd imagine something like NATO still forms in the aftermath, but the Europeans and Canadians might be wary about partnering with someone that evaporated their own city with their own wonder weapon.
The US is still the one who gets the most out of WWII per OTL, but it's going to have an interesting psychological profile because of Honolulu. Will they make more nukes? Probably, since the Soviets are still going to want them. But they'd probably be less bellicose about using them. Also, a good chunk of that postwar boom is going to go into rebuilding Honolulu, and that will dampen the euphoria we saw in reality... especially once Americans meet radiation poisoning.
As for Hawaii itself, I'd imagine it would still eventually become a US state, but the timeline may be altered somewhat due to this absurd and horrible accident. I don't see Honolulu becoming the capital or leading city; that honor probably goes to Hilo.
Well, that was interesting. Again, just my thoughts by playing this straight.