Are you looking for a meaningful, yet frugal idea to show your Valentine how much you love them? Me too! I'll show you mine, if you show me yours! You could:
MAKE AN 'I LOVE YOU' JAR
Cut hearts out of construction paper and write something you love about your spouse on each one. Mix them up in a Mason jar with a bag of candy (mmm...Hershey's Cordial Cherry Kisses).
TAKE OUT AN ADVERTISEMENT IN YOUR LOCAL PAPER
Then the whole world will know!
BAKE A CAKE

Obviously, the picture was an afterthought. In case you are wondering
who Nathan loved---it was You. Well....not
you-you, but
me-you :)
TELL THEM THEY'RE HOT 
Buy some redhots candy, a plastic fireman hat (dollar store), some fireballs, and more!
HAVE FLOWERS DELIVERED AT WORK

Or deliver them yourself a la singing telegram style.
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.Okay, your turn for ideas. Don't leave me hangin'!