Such a great, romantic idea!! I would be much happier at your house then at a fancy restaurant.
Since I'm a good detective in figuring things out, I'm going to give you some advice to avoid this going wrong. She may not figure it out, but I would! ;)
Make sure you actually forget whatever it is at home! It's best if you just forget your whole wallet! You can't go anywhere without your wallet. Actually leave your wallet at home. If she sees what you said you forgot at home, she'll get suspicious. Now, will your girfriend offer to pay? If so, you'll need to make sure you are prepared to say why you need to go back home.
She might offer to stay in the car while you run inside and leave the car running. Tell her you need to take the keys to unlock your front door. If she insists on staying in the car, you can go inside for a few minutes and not find your wallet. Then come outside and tell her you are having trouble finding it, and bring her inside to help you look for it or to wait inside so she doesn't get cold.
I hope you have a fantastic Valentine's day. It sounds amazing! :) She's very lucky!