Questions only for an activity's participants

Open Events → Events → select your event → Edit Event → Activities → select the activity → Details.

Save the activity, then open its Custom Fields area and select Add Custom Field. Add the dietary restriction or other participant question there rather than on the event-wide Registrant/Guest form. The activity's fields are presented for guests selected for that activity, including a registrant who is participating as an attendee. Preview the flow with both participating and non-participating guests.