At one time it was drilling and tapping for scopes. But today, almost all factory rifles come drilled and tapped, and ex-military rifles are collectors items, so that job is not called for as much. Same with shotgun recoil pads; once a staple of the gunsmith's work, lighter recoiling guns and factory pads have cut out a lot of that. Scope mounting and bore sighting are still common, but even those jobs are now usually of the DIY variety.
For the general gunsmith today, I think more exotic jobs, like machining muzzle brakes, glass bedding and free-floating, and trigger/safety installation and alteration are common.
Due to various pollution laws and OSHA regulations, few small gunsmith shops will do rebluing any more, so that is another bread and butter job that the nanny state has taken away from the average gunsmith.
Of course there is always the brown bag full of parts of the revolver "my kid took apart", but taking on those jobs can be tricky. The customer not only wants you to put the gun together, he wants you to make or find any necessary parts, like the hammer, trigger, and cylinder, and put them in for free as a goodwill gesture. And when you give back the gun with a bill for $75 - a third what the labor cost - he tells you that the "old POS" isn't worth that (you told him that at the beginning) and to put it in a dark place.