Difficult one to answer, since personal preference varies greatly. Cards without sentiments would be practical, but if they're like my mum then she prefers a sentiment on her cards. I would say mainly birthday cards with a couple of 'get wells' and 'thank yous'. You could put an insert in with a simple verse not too sentimental or personal, and they can always add something more personal if they want. You could even put a couple of blank cards in as well.
Jan