The "service" that I get from my independant garage includes :-
Spark Plugs (but depends on whether they need doing at the mileage the car has done)
Check all fluid levels (and top up as necessary), check brakes, electrics, general condtion of car / mechanicals and a road test afterwards.
Anything else that they pick up, attracts an extra charge obviously, like discs and pads etc.
So not much really, and its certainly not something that couldnt be done at home IMO. Not that I'd attempt it these days as I dont have the time etc at the weekends anymore.
I've only ever been to a main dealer for a service once, that was years ago when I had a Vauxhall Cavalier, and at the time it deal fill with confidence that the service was done by a dealer. These days I wouldnt trust main dealers for services etc, I'd rather use my trusty independant garage that knows about cars and mechanicals.