Don't forget to try Banshee at night also. I feel like inverts like Banshee are definitely worth the wait to ride in the front if you have the time. Last season they were assigning rows on Banshee but would often take requests if you wanted the front. The other thing to keep in mind is that it seats 4 across so that wait for the front may not be as long as it looks. I agree that Banshee, Diamondback and Beast are the star attractions here. Diamondback is a very different experience depending on where you are in the train. As stated above, Firehawk is a pretty fun ride but has a painfully slow line. Vortex has a line that can be deceiving. It has almost not queue area so the line can spill onto the midway and appear super long but in actuality is not and the ride has a very high capacity. I wouldn't expect super lines on Friday night, Saturday could be a different story though. So I would just play the Fast Lane by ear.
Something else to consider is the food at the park. Two Cincinnati favorites are Skyline Chili and LaRosa's Pizza. Kings Island offers both but I fee like their food is better if you try it at an actual restaurant location outside the park (and cheaper too). I believe both can be found close by.
Holiday World is absolutely worth the trip. The Voyage is one of my all time favorites and I can't wait to try Thunderbird! Depending on which way you are traveling from Kings Island to Holiday World you could drive through Louisville and also check out Kentucky Kingdom if their season has started yet.
Hopefully that helps a bit. Have a great time!
