I will start simply by stating the things you should NEVER do.
Never buy her something 'practical' unless it's only a part of the gift or you are hiding something better inside of it. No kitchenware, no small appliances, no furniture and nothing that she knows you wanted.
What to do for the day?
Surprises are great. What are her interests? Plan a trip to a nearby town for the day or even a weekend and do the things SHE likes to do. If she loves boating, nature, science, art, seafood and karaoke find places that offer those services. Perhaps you can start the day with a light breakfast and a stroll down a nature trail that leads to a canoe rental place and take her for a stroll on the lake/stream/river. After that hit a local museum for a dose of art and science. Stroll around and find a place to have a snack. Perhaps go back to the hotel to change and head to a Seafood restaurant. If you live near water, it doesn't have to be expensive. Usually places with a pier have very affordable fresh seafood. After dinner, hit the town's Karaoke bar and dedicate a song you will sing just for her. If you are a terrible singer ask her to sing for you -- because you can't live without hearing the sound of her voice.
If you're on a budget, don't worry. Women find it terribly romantic when a man plans a date at home. Take a day to stroll through your town and see the sights and once you are home have her change into something nice -- and make sure you remember to dress for the occasion. Yes, I know a tie is uncomfortable and those dress shoes hurt but do it for her, she will like it! Have her sit down to a dinner you are making. Pick her favorite dishes and have at it. Dancing alone in the living room is also a must for couples. No one is watching so there are NO EXCUSES!
As far as buying the right gift, it's really not that difficult. Flowers may seem overdone but we all love them, especially if you choose our favorite one or find an arrangement in our favorite color. Valentine's themed items are great in small amounts. A candle, a stuffed animal, a balloon -- but don't over do it. It makes it look like you don't know her at all and you just 'picked something up' out of habit. I also find that live flowers are more romantic than a cut bouquet.
If you want to do something really special make something for her yourself that she won't forget. Perhaps you both still save your first love letters to each other (or e-mails/text messages) - Having it printed up on special paper and framed along with a keepsake or two from the early stages of your relationship would be a nice way of saying 'I still love you this much'.
The important thing is that you show her that you KNOW her. These things tell her you are listening and that you care. Gifts on Valentine's day need to be FOR HER!
This also applies to the early stages of a relationship. You can still do special things without going overboard -- take your best guess or flat out ask her what she likes and "LISTEN". If you are not sure even after you ask her, you can always give her the gift that every woman loves -- a day of pampering. Do an at home SPA for her with you as her personal servant.
For the other women in your life -- Mom: Something for the home that tells her how much you love her is always the perfect gift.
Little Sister, Niece or Daughter: Something age appropriate and cool. Small kids love those musical stuffed animals that dance and sing or even a plush heart pillow. Older girls may like something like an MP3 player, Spa Basket, Candles or a great book.
With that I leave you and hope that you all have a fantastic Valentine's Day filled with love and appreciation.