Good Question,

You should have the spark plugs changed, they are easy to get at (
especially the rear ones) with the upper and lower manifolds off.

The o ring gasket which is on top of the oil pump. This is place where a
distributor would go, (don't use them anymore) again easy to get at when the
manifold is off.
You should also have the 2 small rubber crossover hoses from the coolant
bypass to manifold. The curved hose is expensive but they are both over the
exhaust pipe and dry out. I did not have these hoses replaced when the lower
manifold gasket was done, they started to leak and  they are a real pain to
do after.
And if the car has never had a new thermostat this is the time to have one
put in.

As you might have guessed, been there done it.

Make sure the mechanic uses a really good lower manifold gasket set, there
have been some mods done to the gaskets over the last few years. Cheap is
not the way to go with this repair.
Hope this helps.

My Malibu 98 is having its lower intake gasket replaced.  While doing so, is
there any other gasket that I should have the mechanic replacing?  Or
perhaps: are there any other gasket that must be replaced as part of the
procedure?

Thx,

Guy