Seeing Hearts

Cut heart shapes out of construction paper, then cut each heart shape in half.
  Set out the heart halves and small, rectangular mirrors.  Invite a few children
 to join you.  Challenge them to find a way to use the heart halves and mirrors
 to create whole hearts.  If needed, show them how to
 hold a mirror along the cut side of a heart to do this.

Seeing Red

Make red eyeglasses for the children to wear on Valentine's Day. Cut heart
 shaped frames out of red poster board.  Glue small squares of red cellophane
 over the backs of the eye holes.  Then attach pipe cleaners to the sides of
the frames and bend them to fit over the children's ears.  Encourage the children
 to describe how colors change when seen through their red eyeglasses.

Magnet Fun

Provide the children with a variety of magnets to play with.  Explain to them tha
t magnets attract each other and stick together just like friends do.

Valentines For the Birds

Let each child use a valentine cookie cutter to cut a heart shape out of a slice
 of stale bread.  Have the children spread peanut butter on their hearts and
 then sprinkle on birdseed.  Insert a pipe cleaner through each heart to make
 a hanger.  Help the children hang their valentines for the birds on  a tree outside
 a window in your room.  Let them also spread the leftover bread pieces on the
 ground around the tree.  Encourage the children to watch for the birds that
 come to enjoy their valentine treats.

Variation:  Make a bird feeder by cutting holes in the sides of a half gallon
 milk carton.  Let the children paint the feeder with a mixture of red powdered
 tempera and liquid soap.  Have them decorate the feeder with valentine stickers.
  Then place birdseed inside the feeder and hang it outside a window.

Listen to the Heartbeats

Talk with the children about how the human heart works.  Then give them a
 stethoscope and let them use it to listen to one another's hearts.  Ask them
 to try describing the sound that the heart makes as it beats.  Let them listen
 to one another's heart while sitting quietly, then after doing jumping jacks.
 What differences can they hear in the heartbeats?  Talk about how our hearts
 need both rest and exercise in order to stay healthy and strong.

Colorful Carnations

Materials Needed:
Three fresh white carnations,
warm water,
knife,
food coloring

Procedure:
Talk to the children about last valentines day (or a long time ago)
 and the present of white carnations you received. They were beautiful
 flowers, but you had wished for some red flowers, not white. "At the
 time I didn't know what to do, so I kept the white flowers and gave them
 plenty of fresh air, and new water every day. Nothing happened except the
 flowers got old and died. This year I found a way for the flowers to change
 color. Should we try it?" First give the flowers a new cut on the side. Then
 fill the vase with warm water and food coloring. Put the flowers in the water
 and wait. (try with different colors and wait for the colors to creep up into the flower).

Pre-K Fun Theme Pages are for educational reference only! 
No copyright infringement is intended.
I do not claim any of these as my own ideas.  
They are shared from friends and fellow group members.  
Thanks for sharing all your great ideas!