The typical small mods like better flowing filters, under drive pulleys,
 better flowing exhaust, roller rockers, etc will improve mileage by
improving the efficiency of the engine or reducing parasitic drag and
friction.  It won't be drastic but it should be somewhat noticeable.
This assumes you drive conservatively.  Improving airflow into, and out
of, a basically stock engine almost always improves gas mileage.  Also,
look into the under drive pulleys.  They are a good bang for the buck
mod that will also save gas.

Personally, my advice is to leave the car stock if an increase in
horsepower isn't your main reason for the modifications.  You will see a
relatively small improvement in gas mileage (2%-5%)which, IMO, isn't
worth the cost of the mods.  Plus, these days, intake systems on newer
cars are pretty efficient and you may gain nothing at all with an after
market setup.  You are probably better served by installing a more
efficient exhaust system.

Before you buy anything, tally up the costs and figure how much gasoline
that amount of money will buy you.  With intake and exhaust mods you
will spend about $400-$500 dollars at a minimum which is 3,500-4,000
miles worth of driving at 22 mpg.  To recoupe that money from gas
mileage improvements (assuming a 5% increase in mileage) means you would
have to drive 80,000 mile to break even, cost wise.  My advice, and
personal experience, has shown me time and time again that modding a car
is very rarely justified economically.