You can make model on TRANSPORT SYSTEM IN PLANTS.
you can show show movement of water and minerals using red LEDs
and blueLEDs for movement of food.
Firstly take a thermocole ,paste chart, draw a tree.Now to make it attractive show the movement of water and minerals using red pipe LEDs and movement of food by using blue pipe LEDs .The diagram of transport system in plants is given in Class 7th book.
You can also make a modle on electric circuit .You just need to connect bulb,wires,switch ,cell etc.