Hello sluggo1010,
Sounds like you want a smaller gas tank, that acts like the gas tank you have now. Pump clicks off when it is "full", gas gage reads "full" when it is "full, etc.
If you want to use things that float - like the play balls, if they are compatible with gas - I'd suggest some sort of screen around the fuel pump and fuel sender so they don't interfere. You might have problems with being unable to fill the tank very much at all as a ball floats up and plugs the filler, triggering the auto-shutoff on the gas station pump. In a non-repeatable fashion, as they move around when driving.
You could put in a smaller gas tank with about the same amount of labor you're looking at to put the balls in. So long as it has the same "type" of fuel level sender (resistive, capacitive, other) and is also fuel injected with similar fuel pressure/volume requirements, the donor tank's pump and sender can be adapted to your Civic.
Or, for a lot less hassle, you could buy a Scangage/Ultragage/Mpguino (whichever will work on your car), calibrate it, reset fuel tank size to whatever you want to put in at the pump, and then just put less fuel in at the pump.
If you want a more violent, cheaper, more dangerous way: take old tank out, drain and dry it and maybe purge it with something non-flammable, mark on it where the fuel level sensor is, the inlet and vent pipes that extend inside, and where the fuel pump/pickup are (so you can miss them), and make it smaller with a sledgehammer. Make sure the spot under the fuel pickup and sender stay the same depth, and the spot under the pickup is the lowest point - with no trapped low points other than that. Beware sparks and don't put a hole in it.
