If you're reasonably sure your sweetie is going to say yes, call the theater in advance to see if you can propose on stage.
Look for a play or musical that has a weddings or love theme.
Another idea is to buy a bag of conversation hearts and find one that says "Marry Me".
Don't forget to buy tickets early as Valentine's Day is a popular evening.
A variant on this is to buy an ad at your movie theater that will show before the previews begin. You'll need to ensure that you get there plenty early!
Say it with Candy Buy your sweetheart a heartshape box of chocolates and replace one with the engagement ring. ) may be surprised that you've gotten her such an inexpensive gift, but if you can get her to open it, the surprise will be on her!
On a Walk/Drive/Sail: Take some time to find the most romantic spot near you, then lead your Valentine there in a leisurely way, talking about why you are in love and why your relationship is so special.
If you're planning a Valentine's Day proposal, check out these ideas for making the moment special and memorable.
Proposing in Front of a Crowd: Many couples go to the theater on Valentine's Day.