My children love to pretend to be leprechauns. Here are a couple of ideas:
*Provide a green hat or vest or shirt that they can wear.
*Decorate the play area with green: pillows, blankets, blocks, streamers, etc.
*Provide a pot of gold that they can hide.
*Make a rainbow or have your children make a rainbow.
*Have your children talk like leprechauns.
Any other ideas?