Fast hot water - tankless heater as backup or recirculator
We have a circulation pump that was in the house when we bought it.  The instant hot water is nice, but it is a huge energy waster, as you suggested.  Our solution was to put switches on the circulation pump.  It's plugged into a receptacle near the water heater, so it was as easy as making that a switched receptacle.  We have switches (timer type that turn themselves off soon after you turn them on) next to the shower and by the kitchen sink.  It only takes about 30 seconds for the circulation pump to get the hot water to you, so we just turn it on while getting ready for the shower (or while stacking the dishes).  So convenient, but without the wasted energy or water.