You'll need a set of pre-cut wooden numbers and a pack of small self-adhesive magnets (both available at art/craft-type shops). Then... well just decorate the numbers however you like, let them dry, then stick the magnets on the back (we found we needed two or three per number to make them properly magnetic enough to hold the weight). Here are some ideas:
Glitter glue - this is what we did - two contrasting colours per number for added effect.
Fluorescent paint - to make funky glow-in-the-dark fridge magnets.
Googly eyes - paint the numbers and stick on eyes to make little people or animals.
Sticky jewels - use the self-adhesive kind for a completely mess-free project.