I would send cheque or do bank transfer, they might have something specific in mind like a holiday or from a specialist store that doesn't do vouchers.

I always think the greatest gift is to give someone something they want/have asked for. For our wedding work gave me money (had not requested anything but they knew me well enough not to buy anything) and we got some of my artwork onto canvas for the wall (we will love those forever). I also think it is a way of showing your respect for them in that you are happy that they will put money to best possible use for them.