First you must think about the woman and what you know about her.
Take into account her interests and lifestyle but don’t get too hung up on that. Just be aware that a big box of chocolates may not go down well if the woman is on a diet – and fancy undergarments would be too familiar for a woman you don’t know that well.
Think too, about what the thank you is for – the gift must be well balanced with the deed or favour that was done. It would be really bad form to make your thank you out-do the thing for which you want to say thanks.
The woman would be embarrassed and may not offer help again as a result. If you are a man, she may even think you are being too forward.
A present that is too large is worse than a very small one in the case of a thank you – in fact that is something to avoid in most circumstances.
There are a few things to fall back on that are well – received such as flowers. No woman tires of having flowers and they can be anything from a little posy to a flamboyant bouquet – or even a little pot plant. Just be sure that the size and apparent cost is fitting.