At the cheap shops there are always little party bits & pieces with animals on them, if you don't want to buy 'themed' things buy plain plastic table cloths and pick up the cheap $2 packs of animal stickers and moo can help 'decorate' for his lil bro's party

If you can find some big posters of animals you could get some big pieces of cardboard and make like a pretend 'fence' line the posters up on a fall or fence and it can be like walking through the 'zoo'.

For little kids you could print out coloring pages of random animals, get some cheap packs of pencils or crayons and they can all color their own pet.

Umm prob not great ideas but ideas all the same... will try and think of more but