It all depends on how much you want to spend.
If it is an automatic get 4.10 gears, a manual get 3.73's That will get you into the powerband a lot faster.
Put a aftermarket catback exhaust on it and a mid-pipe. Good for about 10-15 hp.
Install a GT-40 heads from a Ford Explorer. It'll allow better airflow.
Install a Cold Air Intake. Good for about 5hp.
If you have more money, go forced induction. Install a supercharger or Turbo. Or, just install a wet nitrous
Cams and ported heads will also help a lot. Maybe around 50+hp.