You’re right that wheels are a good place to start. Monoblock wheels with performance tires are a fun upgrade for the Navigator.
An ECU software update can improve performance, since the manufacturer didn’t intend for the Navigator to drive aggressively.
Want something more advanced? You can boost power output from the powertrain with the HPE600 package from Hennessey Performance. Upgraded intercooler piping, a high-flow air induction system, and a stainless steel catback will help it all work smoothly.