What to do on [Valentine's Day] depends on what stage you are in. If you are in full blown WAS mode with little to no contact then any gift is pursuing. I like the ideas of doing something with your kids, helping someone else or GAL for you.
The first stage of reconciliation is getting rid of negative feelings. So unless you are having consistent positive interactions then stick with the advice above.
Second stage is becoming friends again. Here I would do something that shows you understand who your spouse is - 5LLs, music they would enjoy, a book, fix something around the house, etc. A gift that shows you have more than a business connection with them. A gift that shows a emotional connection but not romantic. Show them you understand them and that they matter to you.
Third stage is re-igniting the romance. If you are in Newcomers then you aren't here.
For the curious the fourth stage is re-commiting. The is when you both have the marriage you honestly wanted.
Me: 44, Wife: 39 M: 17 years T: 20 years Bomb on 08/25/09 1/13/10: MC started 1/28/10, 2/8/10: More bombs 8/28/10: Wife moved out No talk of D, no movement