You need something romantic for a woman in her 40s? I'm with Gus's mom on the flowers and a love letter.
If you poke around the cnn business site, you'll find a piece on "lovebooks" or something like that - custom scrap books you create online. They might be a way to help you with the love letter thing.
I just googled the details for the 20th Anniversary.
Traditional observation: china
Modern US observation: Platinum
Alternates: lily flowers and emeralds.
Those are the traditional ways to observe this particular anniversary. Since I'm partial to jewelry, a custom commissioned ceramic ring, or maybe something with an emerald could be lovely.
What does your wife like? That might help us point you in the right direction.
Pick a really nice restaurant, find a nice couples bar/dance club and book a great hotel nearby or within a short drive to make it spontaneous, no over night clothes you won't need them. Get a small piece of jewelry so she will have something to remember the night with and give it to her in the room so she will have something to wear to bed.
Get the tooth brush from the front desk in the morning.
Sacrifice. Depends on your own circumstances and finances for the rest. Basically, you need to give her something she wants that shows you are willing to sacrifice something you want to give it. Depending on your circumstances that might be a trip to somewhere she wants, may be giving her some jewellery or something and to pay for it you go without some luxury you wanted (power tools or something), or it just might mean giving more of your time. You've been with this woman 20 years, you know her better than anyone. No sense in giving her something someone here would want or do, we are not her.
Think if you were her. What would you want?
Lame answer I know. But I don't think throwing out generic gift ideas is the way to go.