There are several different versions of this idea around, including a commercial variety called "Resurrection Eggs." I am not certain where the idea originated, but it may have been from the Real Learning Forums. The name of "Paschal Mystery Eggs" definitely comes from Jenn Miller's post on this thread in that forum, which also gives details of a longer version.
Here is the version that we use. Get 12 plastic eggs that can be opened and filled, and a regular dozen-sized egg-carton. Inside each egg place the listed object together with a slip of paper with the corresponding Bible verse. You may want to also include an explanation of the significance of the object, or simply discuss the significance when you open the egg. Number the eggs from 1 to 12, and place them in reverse order in the carton.
We open one egg per day beginning on the Wednesday before Palm Sunday and ending on Easter Sunday.
Fill the eggs as follows:
Day 12: Mark 12:22
a small cracker or piece of "Matzoh" to symbolize the bread & wine of the Last Supper.
Day 11: Matt 26:33
Feather or picture of rooster to symbolize the rooster that crowed 3 times.
Day 10: Matt 26:33
3 dimes to symbolize the 30 pieces of silver that Judas received to betray Jesus
Day 9: Mark 15:17
Thorns to represent the crown of thorns (rose thorns work well)
Day 8: Matt 27:31
Nail to represent the nails on the cross
Day 7: John 19:17-18
Small metal or paper cross represents the cross on which Jesus was crucified.
Day 6: John 19:23-24
A die or couple of dice to represent the lots that were cast for Jesus' clothing.
Day 5: John 19:34
Toothpick or toy lance to represent the spear which pierced Jesus' side.
Day 4: Matt 27:57-60
Piece of white cloth to symbolize Jesus's linen burial cloth.
Day 3: Mark 16:1
Cinnamon stick symbolize the burial spices.
Day 2: Matt 27:62-65
Rock to represent the stone over the mouth of the tomb.
Day 1 EASTER MORNING! Mark 16:5-6
This egg will be empty to symbolize the tomb that was empty because Jesus was risen!!