I think it depends how much ice you have avalaible to you, or could have available.
- Surface area / volume, show how much more slowly large pieces of ice melt compared to the same volume of small pieces.
- Have a container with a large volume of floating ice, mark the water level and show how it doesn't change while the ice melts - already floating ice will have no effect on sea level.
- Have a slope of something covered in ice leading to a container of water to represent an ice shelf on the land, as this melts, the water (sea) level rises.
- Do the melting in water things at different temperatures, fridge, cooler room (outside?) warm place - draw a graph of temp v speed of melting.
- Do you have one of those mist generator things you put under a small amount of water? If so you can use it over a mound of something to represent the Antarctic continent. As the mist is generated and falls down the slope, it is acting like a catabatic wind.
That's all I can think of for now, let me know if you want anything specific and I'll see if I can think of something.