Organize a sleepover at your local library. With the library’s permission, gather up a group of friends and advertise the sleepover. Have book lovers bring their favorite sleeping bag and bookmark for an all-night read-a-thon. Bonus: make it a fundraiser for the library by charging an admission fee or having people get pledges for pages read.
Write a love letter to the library, or your favorite librarian. Make it heartfelt, expressing how much you appreciate the library—or, if you know the person well, make it as silly as you like. Embellish it with plenty of lace, red ribbon, and other Valentine-y decorations.