I used my first attempt at freezer paper stencilling to make an Octonauts T-shirt for my little boy. We used fabric pens and basically did a lot of scribbling, it worked really well!
There are loads if tutorials on how to make a freezer paper stencil, the hardest part was finding some paper here in the UK. It is not the common household product which it seems to be over in the USA.
Anyway, it seems to be a big hit as he is still wearing it! If you know of any quirky stencils anywhere I would love to know, particularly boy ones!