When a tune-up is mentioned, a lot of people think of an engine tune-up. When I worked as a mechanic, the things we would replace in a tune-up were:
plugs, wires, cap, rotor, fuel filter, PCV valve, air filter... these were the most common.
Sometimes we would do other things at the customers request, which include things that George already mentioned up there like belts, Coolant flush, grease fittings, service brake and power steering fluid... Then as he also mentioned you can go as far as doing your bearings, flushing the rear diff., Its all really up to you, but I'd say that all these things pretty much cover everything you need to worry about.