One good surprise to put in children's Easter Eggs is a few LEGO pieces. You can purchase a small LEGO set and divide up the pieces, placing them in hollow plastic Easter eggs. Once children find all of the eggs, they can use all the LEGOs to assemble a model.
LEGO typically makes small Easter-themed sets, but any set that is not too complex works well. Avoid having too many eggs for the children to find, as they may lose track of some. You can number the plastic eggs with a permanent marker to make sure all of them are accounted for. You should also keep the box of the set and any instructions it comes with to help the children put the pieces together correctly.