Something simple I saw done years ago in a restaurant, and have meant to do ever since: You have to have a set of the common basic glass salt and pepper shakers with tin screw-on caps:
Take off both caps; take enough out of each shaker so the contents just reach past the edge of the lid; press wads of napkin on top of the contents; on top of those, put salt in the pepper shaker and pepper in the salt shaker; put the lids back on. The victim can plainly see what is in the shaker, but that isn't what comes out.