How bad of mileage are you getting? Thats really the key to how much you can improve it easily.
The commander I drive came with my girl, lets call them a package deal. She never maintained the poor thing. It was getting an average MPG of.....10! Mostly city.
I changed all fluids, air filter, plugs, PCV, seafoamed it and a couple other little things and we are up to 13.6mpg, still mostly city. Its also more responsive than ever. It had 113,000 miles when I did all that.
Not going to bother doing any intake/exhaust mods with fuel consumption in mind, it would take years to make the purchase pay for itself. And thats assuming you can keep your foot off the floor even with all the cool, new sounds its making....