Depends how deep your pockets are, really. I could rhyme off a list of about 20 items that should be checked. However, these are the important ones:
Spark Plugs: if you haven't replaced them in the last 20-30k, do them. They're usually about 3-8 bucks each to buy, and you can and should change them yourself. It's really easy, don't waste your money on a mechanic for this.
Fluids, Tranny & Diff: again, if you haven't done transmission or rear differential fluid recently, now is a good time to do them.
Fuel Fitler: the part is cheap, but it'll probably cost you an hour worth of labor at a shop to replace it.
You might want to also have the following things 'inspected' and addressed if necessary:
-Inspect the air filter, clean the MAF
-Check the battery
-Inspect cooling system, flush if necessary
-Inspect brakes, may be time to replace brake fluid
That's about it. Really if you do the first couple items, it'll make a huge difference, and won't cost a lot. Plugs will cost you 40 bucks max if you do them yourself. Transmission and rear axle fluid can be done at any old Mr. Lube or whatever, and won't cost much (probably less than 80 for both). Give the MAF and maybe TB a little clean, make sure you have a good air filter, and toss in a bottle of injector cleaner for good measure, and you'll be running sweet.