I thought, being a girlie girl I can give you my opinion about a VD gift. First off...DO NOT and I repeat DO NOT BUY HER UNDERWARE! That must be the dummest idea ever! That gift is saying "SEX" and even though I , as a very sexually starved woman would take it, given the sex implication, I still would think is immature. If your wife is the LD one, she will see it as an aggression.
On gift giving for a woman, the more personalized the better. The gift certificate on a Spa is not such a bad idea, but maybe get her a SPA package, actually choose the one for her. For example, if she likes feet massages, then get her one that includes a pedicure and a massage. You get my meaning. You can also accompany it with something that is a little more personal, like a CD you burned with all her favorite song. Itunes sell individual songs for 99 cents or her favorite something, even if it is small. On the card part. Just buy one without any words in it and write a couple of lines telling her you love her.
I know you say that your good intentions can go to hell in any second if she start acting badly towards you, but you know what? sometimes you have to love uncondicionally. Buy her the VD gift and card no matter how good or bad she behaves. You never know, it can turns things around.