In short:
1. Make spoilers illegal.
2. Allow fan cars.
3. Don't use the fastest tires.
1. Make spoilers illegal: Spoilers loose down force due to turbulent air of the car in front, which makes overtaking a lot more difficult and makes the entire sport more boring. In karting a lot more back and forth overtaking happens because there are no spoilers. That's why Lewis Hamilton thinks it would be the worst idea to increase aerodynamic down force next season. A more radical step which I would take would be to outlaw spoilers all together.
2. Allow fan cars: The human eye is not good at perceiving strong accelerations of objects which are already traveling at a high speed. The spectators don't really see the strong 5g braking since it only takes place at very high speeds. Also in slow corners the cars look very slow because there is no down force. Also the acceleration at the start could be much more impressive if down force would be available at slow speeds. A obvious solution to all of this would be to allow power generated down force similar to the Brabham BT46 fan car. The safety would still be high since the rules can be adapted such that the down force does not increase additionally with speed. And increased speed in slow corners would not be much of a safety hazard if the tracks are adapted a bit.
3. Don't use the fastest tires: With today's very small slip angles the cars look as if they would be running on rails. It is very difficult for the viewer to see how the car is behaving physically. If the cars are driven at larger slip angles, more steering corrections and oscillations of the angles can be seen, which would make it a lot more exciting to watch. Such driving styles are going to be used if the tires have a larger optimal slip angle. Also the tires should be larger or the cars lighter to allow the drivers to use more aggressive driving styles without worrying about wasting the tires.