Wirelessly activate devices using light weight and low-power solution
What about a photodetector and transistor as the receiver - the transmitter could then be a torch which have to shine on a device to switch it on. This is the simplest solution I can think of.

If you are thinking higher tech communications the lowest power and I suspect lightest RF solution might be a the Texas instruments CC430 Microcontroller which has integrated RF. You could of course remove the packaging to reduce weight.