Flashing light circuit
Look for  something like http://www.ebay.co.uk/itm/3mm-FLASHING-WHITE-Ultra-Bright-3mm-LEDs-pk-of-5-/360320012646.  You can run it directly off a 9V battery if you add a 285Ω resistor.  Just get the resistor value closest to and higher than my value as they come in all sorts of values depending on what range you can buy.  You'll want a little 9V battery clip too to connect on the wires as soldering to a 9V is sometimes hard, and it makes it easy to change the battery.

This is absolutely the simplest arrangement.  If you can't get a flashing LED then you're looking at soldering up a more complex circuit that probably isn't worth the effort unless you're into electronics and stuff...

PS.  Do you want a switch too?