If Anki is like my wife, then this birhtday is a "landmark". She'll be 40 and now she is old. Any gift that indicates that she's getting older (cosmetics, spa, etc) will backfire.

Get her something that shows you still find her desireable (lingerie) and wanted (send kids off to g'ma for weekend and check in to a romantice retreat for the weekend, with a small gift (jewelry) for a rememberance).