A cash envelope or a gift card definitely useful, but sometimes seen as an impersonal present. You can avoid this faux pas by adding a thoughtful note with it.
Food52 spoke to Diane Gottsman and Amy Alkon, both of whom write about good manners. Gottsman and Alkon were unanimous in saying that everyone appreciates getting cash. But they added:
The key is to write a thoughtful note to accompany it. (So not just one of those small note-less name cards, but a real card that you write in!) While Diane warned that giving a gift card to someone you're very, very close to might feel "cold," Amy suggested that so long as you write a note expressing why you got it for them, and that it was, in fact, for a thoughtful reason, you're in the clear. "Take some time writing that message!," she said.
There is no template to follow for this. It's about being personal, and meaning every word of what you say in the card.