If you have younger siblings, you could give your mom and dad a night out... while you watch the kids for them. Also, if you can get some pictures of when they were younger, or pics of them while they were dating, you could get a nice collage frame or photo album and give them a trip down memory lane. You're sweet and thoughtful... they've already got the best gift of all... you!
take them out to eat, chocolate, a roze, a anniversary card, a nick nack something like a decoration, a candle, something long lasting. depending on how much money you make ect. buy something online, search for valentines day top sellers. get a mug themed for valentines day. a gift they can both use would be the best, look at there hobbies, mabye a picture frame themed for an anniversary.