We prefer restaurant gift cards.
A personalized sign is a nice idea, if they don't already have one, and you know what their taste in such items is.
My wife loves plants, but we found they are not 'wanted' if we visit national parks, etc. They need to be kept inside at all times. For folks who travel a lot there can be restrictions on plants. Our friends currently on an Alaska caravan were told to not have any plants in their rig when they cross into Canada. We gave them a plastic plant.
While there are all kinds of great gifts, make sure it is something they don't have and would like. Otherwise, stick to gift cards.