How do I make our toilet flush less vigorously?
For adjusting the water level in the fill tank, all of the toilets I've come across are adjustable, there is no need to put something in the tank to offset the volume.

There will be a float of some sort that is attached to a valve.  To have less water fill the tank, you want to adjust the float to sit lower in the tank, thus less water is needed to push the valve closed.

For the two fill mechanisms I'm familiar with:


For the float arm style there is either an adjustment screw or you can bend the arm.
For the newer float on the pipe style, see here. Again, simply the attachment between the float and the top should have an adjustment, move the float lower in the tank so that a lower amount of water pushes the valve closed.