This is a video showing where all the secret targets are located in Archery in Wii Sports Resort.
You have to aim perfectly for the arrow to hit it.

There is a secret target in every stage of every difficulty level.
If you hit all the targets in every level you get a stamp called "The Secret Target"

I found all these by my own. I did not watch any other videos or cheats or anything.

I want to thank Barolb for helping me with some stuff :P

Thanks everyone for giving me over 200 000 views :O