One thing to consider is that whatever you do for one employee has the potential to get around to other employees, no matter how much you or this one employee keep your mouths shut. So you want to give something that, if another employee does as well in the future, you wouldn't mind giving to other employees who achieve the same level of competence.
Then you run into the problem of employees who are almost but not quite as good expecting similar bonuses or gifts, and those who aren't nearly as good but who think they are demanding similar bonuses.
So whatever you do for this one employee, you need to be willing to do for any employee who shows a similar level of dedication to the job and work ethic and results.
I read a study somewhere that what most employees want, up to a point, is more money. After that point has been reached, they want more paid time off.
So, my first thought on reading the OP was to give this employee a promotion and raise, because that is the typical way US companies show appreciation.
If that won't work for your company, then maybe a special bonus, cash or gift card.
Or maybe "bonus" days off. Not a permanent thing, but a one-time deal where she gets a few more paid days off this year.
I'd tie whatever you decide to some specific achievement she's made--certain amount of sales, increased productivity, an innovative idea--so that if you get complaints about how Sally got more days off, you can point to specifics as to why Sally got a special bonus this year. "Anyone who can bring in $XXX,XXX in sales/complete XXX,XXX gizmos per month/other outstanding *measurable* achievement will get exactly the same bonus." Gives the other employees something to shoot for.
Other ideas you could consider might be paying for her to attend a conference in your field, or paying for a course in a related subject, or sending a masseuse to her house every week for a month for a neck/shoulder massage.
You could even give her a choice of two or three options that are roughly equal in value. Could be two extra paid days off, tuition reimbursement for a short course in her field, or a gift card to her favorite place (you give a specific dollar amount for the card).