For breakfast, consider making heart shaped toast or pancakes - which is always a hit with kids, too. If you're in a hurry for time, make the pancakes ahead of time and re-warm them in the microwave. To get the heart shape, use a large heart shaped cookie cutter. If you don't mind food coloring in your foods, you can easily add a drop or two of red coloring to the batter before making them. Kids love having a heart shaped sandwich, so consider that for lunch.
A handmade Valentine's day card is simple to make, and can be a complete hit, even if it's just a folded piece of paper with a heartfelt message inside. If you need ideas on what to write, hit up the internet for lots of romantic poems and sayings.
For a whimsical romantic idea, pick up a box of the little conversation hearts, and stash them all over the house. (Make sure you remember where, so you don't end up with critters having a feast down the road!) An "I love you" placed in a briefcase or lunch, a "hug me" placed on the dinner plate, and a "Kiss Me" on the pillow can lead to a very romantic day, at a cheap price. ;)
The dollar store is your friend when it comes to inexpensive candles. Purchase a few of these, pull out a pretty cloth tablecloth you rarely use, break out the fancy china you rarely use, and any meal at home feels like a 5 star restaurant. If you're pressed for time, you can easily pick up food from your grocery deli, or create a quick masterpiece at home. There are plenty of recipes found online, from beginner to advanced. If you're eating with children, consider making a heart shaped pizza together and letting the kids top it as they wish, using red ingredients like red peppers, pepperoni, and tomatoes.
Shhh... don't tell DH, I picked up a few bottles of sparkling juices on clearance from New Years for us, and I ordered some nice grass fed steaks from our local butcher - a little more pricey, but cheaper than going out. I also put aside some nice dark chocolate bars for dessert that we can share together. We'll have dinner after the kiddo is in bed. ;)