You could use a pair of Nordic nRF24L01+ 2.4GHz transceivers like these http://iteadstudio.com/store/index.php?main_page=product_info&cPath=7&products_id=53 which can probably handle 50m outside, or the 1Km range high power version http://iteadstudio.com/store/index.php?main_page=product_info&cPath=7&products_id=419.