Give the teacher something that she can eat. This could be a batch of your famous chocolate chip cookies, gourmet spices that she can use when cooking or a fruit and cheese basket. Find out what special foods she likes and whether or not she has any dietary restrictions.
It's no secret that teachers spend a portion of their own income on classroom supplies, so don't be shy about purchasing items that she can use with your child. These could be some practical items, such as tissues and antibacterial hand gel, or more creative items, such as craft supplies and stickers. Ask her which supplies she's running low on and purchase from that list.
Rather than spending money, offer to volunteer your time. You could come in one afternoon to clean the classroom or help her to prep for a craft. Some schools even allow parent volunteers to come in and work with students on activities so that teachers have more time for things like lesson preparation and test grading. Ask her what you can do to help her in the classroom.
A gift card may seem more impersonal than a "#1 Teacher" ornament, but you can be sure that she'll use your gift on something she wants. Give her a general gift card valid at any store, or find out what her favorites are and choose a gift card from one of them.
A teacher's work is often under-appreciated, not only by parents and students, but by school administrators as well. Write a letter that details a special way that she has helped your child and give it to the principal. Give her a copy of the letter as well, so she knows how much you appreciate her efforts and that you've commended her professionally.