you cod also make an 2 trasistor bistable multivibrator (aka. oscilator)
and put an LDR in so that LEDs blink acording to the light on the LDR.
you shod also get 555 timers (there are a lot of thing you can do whith them)
I also made once an realy simple metal detector.It has an trasistor,cap,resistor and an trasformer coil.It worked on inductance highering of the coil.It started to make an higher tone beep wen you put the coil on somting iron.nut it cod only betect and iron object on 2-3 cm