For me it's about spending time with each other.
If you want to be together the only thing that will strengthen your relationship is time doing things together.
It's not a one day thing its everyday

Money in this sense is nothing.
Chocolates make you fat
Flowers die
Undergarments can cause great distress
A meal can be just as romantic (or naughty) at home when you both cook.

A card can reveal what that other person is feeling but they can't find the words themselves to say.

Ok I sound cheap but to me it's the person himself not what's in his wallet.