If you are having a special dinner for Valentines Day tomorrow that will also include the kids, here's an idea I came up with this morning while talking to a friend of mine who wasn't sure what they could do to celebrate.
Have a nice dinner, but include some basic etiquette lessons on setting the table, and the proper manners to use at the table for the kids.
Here's one tip from an educator, it helps if the children are also taught how to pick up after themselves, to eat within the confines of their plates (they should be able to do this by the time they reach Kindergarten), and to eat first, talk later. We have a hard time in schools getting the students to eat their food, as they are too busy talking. Before they know it, the bell has rung and they've only gotten a few bites to eat. Getting them in the habit of eating first and talking later will help a great deal for when they are in school.
Here's a video that might give you some ideas for older kids.
For younger children, here's one I found:
Happy Valentines Day!