How about making a small birthday cake from Sculpty?
Either buy the correct color or paint it. Cut out a wedge and draw a line on the inside to show the layers. Get those striped paper clips and use a portion for a candle.
A variation is to use those compressed sponges. Cut out a circle and then cut out a wedge before puffing it out. Once dry you can draw a line to show the layers, and then paint on the icing.