Use Wrapping paper
Gift wrapping papers can be quite useful when it comes to making homemade Valentine's Day cards. You can simply cut out a rectangular piece from a chart paper and then cover it with a nice wrapping paper. Once the whole piece is covered, you can take a thick bright marker and write down your message on the front.
Pop out cards can be made easily at home just by folding a long piece of paper approximately an inch in width. One end is then pasted to the middle of the page while the other end to a cut-out paper heart, giving the card a three dimensional look. A family picture can also be used in a similar way. You can simply close the cards and put them in an envelope so that they can pop-out when opened.
If you have ribbons lying around the house and a stapler, you can cut out small strips to go with the borders of your homemade card. You can stretch the strips from edge to edge and staple them in place. Glue can also be used but it won't be as durable as staple pins.
You can easily find appropriate and fun images and drawings on the internet if you search for Valentine's Day. Once you select an image, simply print it out using the printer and then use it as the base for your card. You can cut out the image from the printed page and paste it on a rectangular piece of chart paper with glue before writing your message.
Write a letter
If you want you can also take a plain piece of paper and write down a letter for your parents. A complete letter detailing your feelings for them is going to have a bigger impact than a greeting card. You can also decorate the letter the same way you decorate homemade cards.