blue -
i can almost guarentee that the only thing you are gonna have to do on your trip is get the oil changed once or twice depending on how far your trip is.
2nd most likely thing in my opinion, flat tire.
As far as the mixed coolant -- i woudnt even worry about that. just make sure you have some water. In the event your engine over heats you can just add the water (** after the engine cools**) to make your car drivable to the next exit or town to pick up whatever you need there .. there is no sence in carrying too much emergency stuff or you wont have any room left in your car. If you carry a 50/50 mix of coolant you wont be able to use the water for anything else if the need arises -- drinking, washing, cooling off, cleaning a cut, etc etc etc. Water can also be added to you washer fluid tank (not below freezing) in case you are stuck in bad weather and run out of washer fluid. It doesnt have cleaning properties, but sometimes you just need a shot of water to help things go away.
Oil and water are about the only two things that you could possible need on a long trip anyway. If something breaks so severly that it leaks all the fluid at once, chances are you're gonna have to call a truck anyway :-(
Things like a serp. belt wont even do you good unless you have the tools in your car to change it. If you dont ... consider leaving it behind. No sence in haivng a belt if you have to call a truck anyway. Chances are slim that the tow driver will loan out his tools for a lesser price than towing your car to the nearest shop.
--Not that you need a lot of tools to change the serp belt, but my point is -- lets be safe and cover the most likely happenings, but lets not get crazy.
An air compressor isnt necessary if you have a can of that slime stuff -- the can will have enough air in it to get you to a service station -- slowly. But truth be told, a lot of those all-in-one jumper/starter/air compressor/power center units are a good deal now adays.
I feel like im rambling now, but I dont want you to be carrying all this extra stuff that wont be practicle.
Other non-maintenance things are:
If you're going to be driving through any severe climates or things like that make sure you have some personal effects to accomedate you on the trip if need be. For instance: water (see above), sun screen (for your left arm hanging out the window), snacks (to keep you away and alert on the way -- i've heard apples work good, but those dont keep as well as crackers or a granola bar -- stay away from too much sugar, it will make you drousy), gloves, hat, extra top and bottom if its winter.
Garbage bags -- can double as a poncho and are great for the environment. Why? because if you have a garbage bag people are more likely to take your garbage with you rather than A)throw it out the window, or 
leave it on the side of the road. -- Not you specifically of course, but I am a nature freek, OK nature lecture over.
About the knife, from my experience a lot of states use a rule that can often be as simple as "if the knife, when laid across yourr palm, is longer than your palm, it is 'illegal'". Please dont quote me on that -- im not into the whole knife thing, but I have heard that a few times before. Plus, a knife can be used against you if the perp. gets ahold of it. Try taking pepper spray or something like that.
I'm racking my brain trying to think of what else you will need, but chances are you arent even going to use the oil and water.
Have a great trip, and always remember, ENJOY IT.
