Cards are one of those things that I have a hard time spending money on too since they are often discarded and sometimes not even read by the recipient, but I notice people look at a homemade card a little longer and they may even hold on to it for a while.
You could easily make your kids a puzzle out of any paper/cardboard item.
For this one I chose to use the paper doilies we had from our valentine playdate. I used double stick tape to stick the smaller heart on top of the larger heart and to stick the larger heart on to the construction paper.
To add a little flair to it, I sprinkled glitter on the contact paper before placing the card down on to it.
Next I cut out pieces of the puzzle
Here is the stack of 2 puzzles, one for each kid. I used a different background color & marker color for each one in case they mixed them altogether (which I could totally see happening.)