How about you just surprise with a sexy new nighty and some candles. Then maybe a sex toy or something. Get a pretty scarf and think of how and where to use it
Have either of you given the other a full body massage? They are heavenly and totally relaxes a person, which can lead to good stuff.
You know that she is conservative, just start out slow and if she is wondering what's up just tell her that you are wanting to add some spice to your relationship. She might be wanting the same thing, but doesn't want to bring up the subject. You 2 have been married for 20yrs, you should be able to talk about your marriage, and sex is a major part of that.