You just bought that used hooptie Craigslist's been trying to slang for months. It managed a straight line on that test-drive and dad gave it the official thumbs-up following that tire-kick, but the truth is you don't know diddly about that $3000 lump you just drove home in.
Which is exactly why you ought to assume the worst. Pretend that oil hasn't been changed since September. Of 2004. Act like that timing belt's never seen daylight and that the transmission fluid's the same stuff that the guy on the assembly line squirted in. Assume the worst and you won't have to go wondering whether or not that water pump'll fall apart tomorrow or two years from now. Cover the following areas and the chances of you encountering all sorts of expensive problems just got a whole lot better.
THE OBVIOUS STUFF
Engine oil: What that engine oil looks like can reveal a whole lot of history. Do the obvious and swap it out with a fresh filter and an oil change. Clean oil of the right viscosity is the only thing keeping stuff like crankshafts and connecting rods from grinding up against one another in ways you can't afford. Filthy oil that's a quart shy of what your engine says it should be is a sure sign you oughta be skeptical of just about everything else.
Air filters: Like engine oil, a cruddy air filter can be a sign of neglect; nobody's going around flushing out things like transmission fluid when they haven't even swapped out a $7 air filter.
THE WET STUFF
Transmission fluid: Chances are, your car's service manual says that all that transmission of yours needs is a quick drain and a refill. Your service manual doesn't realize that the bonehead who owned that Accord before you never once did anything like that, which means you draining out three quarts of ATF will still leave you with six quarts worth of muck. Here, flushing that whole mess out is your only choice. Most would opt for visiting their local dealership and throwing down some hard-earned dollars on this process. You'll see below that, with the help of OEM tools, you can do it yourself, in your driveway, and save some money in the long run.
Differential fluid: If you've got a serviceable differential, like you'd have with anything RWD or AWD, then you'll want to drain whatever fluids inside. Most differentials don't need to be flushed; they don't have the same sort of inner workings that an automatic transmission does, for example, which means almost all of its fluid can be drained out just by removing that plug.
Coolant: You think opening up that radiator valve and draining what's inside into a bucket is good enough and you're wrong. Most of that cruddy old coolant will be found in the engine block and should be drained from there, too. Look for the engine's drain valve or block-off plug which you'll find someplace on the block's lower half.
Power steering fluid: Flushing the power steering system isn't always necessary. Most of the time, draining and refilling the reservoir a couple of times will cycle in enough new fluid.
THINGS THAT GO SNAP AND POP
Timing belt: At best, a slipped or broken timing belt will make that engine of yours barely run. At worst, it'll squash all of its valves and pistons together in one big tidy pile. You pulling off that plastic cover, looking at that timing belt, and knowing whether or not it needs to be replaced isn't gonna happen. Timing belts rarely show signs of obvious wear, which means replacing it now might save you from that bottom-end build-up you weren't planning on.
Accessory belts: Things like serpentine belts and alternator belts, for example, won't cause the same sort of catastrophic damage as a snapped timing belt might, but not swapping in new ones is just as dumb of a decision. A flung alternator belt can still leave you stranded, and a serpentine belt that drives the water pump can leave you with a blown head gasket.
Cooling hoses and caps: A failed radiator hose is a wet and obvious mess. A radiator hose on the brink of failure, not so much. Replacing the upper and lower radiator hoses for new ones will never be a bad idea.
Water pump and thermostat: You replacing that timing belt and not taking care of that water pump at the same time won't be your brightest move. Even if your engine's got a timing chain or its water pump isn't driven off the timing belt, it still ought to be replaced. You've got no way of knowing it's lifespan, which means starting from square one with a new pump will be your best move. Same goes for the thermostat; with the coolant already drained, there are few smarter ways for you to spend $15.
Spark plugs: Not swapping in a new set of plugs can hurt everything from gas mileage to performance. Not swapping them out ever can make them nearly impossible to remove and can even damage the threads they're screwed into.
Valve clearance: Not every engine's valves can be adjusted, but if they can, it'll do you well to check their clearances. Valves without enough clearance can lead to poor performance and can damage the valves themselves. Valves with too much clearance can hurt performance, too, and will make a whole lot of noise.