The main problems stays when there's is more than two sensors, to allow them to communicate easily and to be identified.
Simply give each their own little unique ID that they send out along with their data.
I would have little, cheap, and low power consumers wireless sensors. My choice is perhaps to use Arduino Nano cards, with the sensor of my choice. To communicate with the main "central", I thought about RF Transcievers from Sparksfun.
You can get away with using even smaller, standalone microcontrollers if your sensors don't need to do much more than collect and send data like I've done here:
ATTiny84 for ~$2 and a RFM12B transceiver for ~$6. You could also spring for a cheaper wireless module like the nRF24L01.