Super quick and easy to set up, minimal mess and cute results. Bend a cardboard tube, toilet roll for example, and fix the top with some tape to keep its shape. If you have some heart shape cookie cutters, they will work too. We also made 2 potato stamps. Some coloured paper, paint and let them at it.
The potato stamps worked out much better, as neither of the younger 2 had the dexterity required to get the roll to print properly. You could finish them off and write messages on with a sharpie as I did in the last image, or you could just use them as decorations rather than cards.