Actually, a coupon is a good idea. If you have a little bit of time and a little creativity, you can make her a 'One Free ----' Coupon. It's easy, free, and your S/O will like that you put a personal touch into the gift.
uhh, can't you just show up with flowers? I mean everywhere sells flowers on valentines day. even till late, surely. Please tell me you haven't seen her all day though. I just picked up a rose on the way home and I was set.
Turn up with flowers. It's expected and it's your obviously only course of action. Unless you think real hard about something fast and you probably wouldn't be asking us if you were good at that......
Gifts are a waste of money, just tell her something she's never heard before or do something you've never done before, she'll like it more and you'll save money.
Go buy tickets to a show and tell her that you were late in getting the gift for her because you had to stand in line for hours to purchas them. If you order them online, then tell her that you were on the phone for hours making transportation arrangements.
My dad used that last minute wonder on my mom every V-Day and she never did catch on. :lol