I'm thinking sensor networks. If you can, then I would recommend to try something using the MSP430 for example, given its low power requirements. Take the microcontrollers and pair them with different sensors (eg., IR, temperature, etc) and an RF module such as xbee [9] in order to create a mesh network [10]. You'll need to work on the protocol and management side too-- a linux box somewhere with a database but that should not be a problem for you. But I realize that is more of a team project given the amount of work.
I mention this because I see some activity in this field and lately I've seen several articles, mostly in The Economist, that talk extensively on how sensor networks and the system that manage them will encroach our daily life. So if you can't figth them, might as well join them.
[9] http://www.digi.com/products/wireless/point-multipoint/xbee-...