What a great question. William Shakespeare's Macbeth illustrates what an overabundance of ambition will do to a person. Macbeth, the tragic hero of the play, finds himself in many different predicaments which would make for "good" memories to be placed into a keepsake box.
His robes of Cawdor--These would represent a time when he knew good from evil. When asked about being king, Macbeth states that he wants to get used to his newest title first. Macbeth makes this connection describing his new title as robes.
The dagger--The dagger represents where everything begins to go downhill for Macbeth.
A pillow--Given that "Macbeth doth murdered sleep" (II,ii), Macbeth may wish to keep a pillow in his keepsake box so that he can remember what it was like.
A bottle of water--This would remind him of the fact that he was correct in stating that all of the water in "Neptune's ocean" (II,ii) would not be able to wash the blood from his hands.
A cross--Although Christian thought is practically nonexistent in the play, Macbeth may wish to keep a cross in order to keep him safe from all supernatural things (witches and ghosts).
A mirror--Macbeth should also keep a mirror in his keepsake box. This would allow him to look inside himself in order to keep himself in check (if he would have survived).
A cauldron/book of spells--Macbeth actually turns out to be far more evil than the witches (who he believes to be the most evil of all of the characters). Therefore, if he ever chooses to take up magic, a spell-book or cauldron would serve him perfectly.