Usually the parent groups (PTA, room parents) organize something so everyone can contribute according to their ability (financial situation, how many kids they have in the school system, etc.) which shares the burden and gives the teacher a larger gift she can use more readily. Usually someone in the group is familiar with the teacher and knows her likes/dislikes.
From my experience as a teacher, the small trinkets are a problem because we get so many of them, feel the need to display them/use them, etc. We definitely don't need any more stuff that says "World's Best Teacher". While any gesture is appreciated, it can be hard for a teacher to drive all over to use up every Dunkin' Donuts card or shop in every department store for which she got a $10 or $25 gift certificate. Remember that almost all teachers have a second job because they don't get paid enough, so if they get too many gift cards which they kind of have to add to, it's a problem. So, if you give $20 at a restaurant, the teacher has to fork over the additional money to go out with a friend or a spouse, so it actually costs her in the long run. I got a $10 gift card to a delicious ice cream place, but it was the other direction and near the school, but 15 miles from my home. I couldn't very well go during the school day, and after school I was doing lesson plans and grading tests. To go during the summer with my husband meant driving 15 miles each way and having to add to the gift certificate just to get a nice dish of ice cream. It was a lovely gesture by the family but it was more of a hardship to me, so I wound up giving it away.
A small package of little things like tea or designer cookies or coffee samples means there's something the teacher is likely to enjoy, and it shows thoughtfulness in putting together a cute little basket or gift bag. So if you don't want to give cash/gift card, do that.
A thank you note is lovely, and if it's from the child (at least if the child "dictates" part of it while a parent writes it down) is truly lovely. No matter what silly thing you child says, write it down. If he says "Mrs. Smith smells good" or "Ms. Jones likes my stegosaurus picture", put that in there. Your child is learning to express gratitude, which is the point of this. Let your child help wrap something (no matter how bad it looks!) or bake something so he participates. Kids don't understand gift cards or even "adult" gifts like coffee, and there's no thought involved from their point of view.
Going forward, if you can get the room parents involved in this, it's much easier to do a larger, joint gift.