I started with the USA license plate canvas that was purchased at Target for around $30.
The giant stick was a special one he carved at Scout camp as was the rabbit fur. The feather he collected (hung with a tiny command strip), a scorpion my husband brought home from Afghanistan and a horseshoe he got from his Grandparents. Hang up stuff that your kids like and have some sort of meaning to them.
Gallery wall tips:
1. Start with one big thing.
2. Don't be afraid to hang non-traditional objects.
3. Hang things with meaning, a story or a memory.
4. If it's your kids room choose photos that make them proud of themselves
5. Just go for it! I sometimes hang my gallery wall before there are even photos for the frames!