We have an old antique wash jug and bowl in the bathroom, the jug always gets filled up from the hot tap while the hot water comes through, and we chuck that into the cistern after a flush.

At least the water does not get wasted, cumulatively it save a bit.

Garden is only watered with rain water from the 5 water butts we have dotted about, never use it to wash the cars as it promotes algae growth on the rubber seals around the windows, mains water does not do that due to the chemicals in the mains water.

Every little helps as they say.