@ColinGross "Can you choose to take an action that the caster doesn't know how to do? E.g. could you have the target cast a spell?" - as Mike said, they could try and fail. A related Sage Advice about using Suggestion to tell someone to "Sleep": https://www.sageadvice.eu/2018/01/21/if-i-use-suggestions-to-tell-someone-to-go-to-sleep-do-i-have-an-8-hour-sleep-spell/
"The suggestion spell allows you to compel someone to pursue a course of activity. You could, for example, tell them to go to sleep. That doesn't knock them out; it causes them to try to go to sleep naturally. It isn't magical sl…