2003 Chevy Impala Taking a Trip

Check your levels (oil, coolant, transmission)
Change your fluids if necessary: every 3000 miles for your oil, 150 000 miles or 5 years for your coolant (if you use GM dexcool) but I'd suggest sooner.

Check your hoses for leaks.
Check your brakes (when was the last time you changed them? Are they squealing)

Check your serpentine belt; if you haven't put a new one in a LONG time (like 60000 miles), install a new one ($20).

When was the last time you change your timing belt?
(I'm not sure what the recommended interval to change it is, but if you've never done it, now wouldn't be a bad idea to change it.)

Check your tire pressure the day you're fixing to leave.
Make sure all your lights are working.

Obviously, if the car has abnormal reaction/behavior that you're aware of, now is the time to investigate these as well.

Finally, have you done a tune up lately? If not, get one done if your manual recommends it.