The basic idea is a bag with a window that is full of small items to find amidst a filler (like rice or polly pellets) The child must move things around to find all of the hidden items.
This web site is wonderful, with pictures (including the one on the right) and clear instructions.
I also like the idea of having a list of things to look for. Using pictures or a duplicate of each item would work well for children that can't read yet.
And if you want a no-sew project you can always fill a zip lock bag with rice and small objects. But I would make sure to seal the opening with duct tape before giving it to a child!