When I worked at a place where some aircraft were on the charter licence and some weren't, the rule was that if the customer came back to the base airport together with the instructor then it was instruction/sightseeing, but if the instructor dropped them off somewhere and left, it was charter, regardless of who was at the controls.
"Can I hire you to take me over to XXX for a couple of hours?"
"We're not licenced for passenger transportation, only local sightseeing. I could take you on a sightseeing flight and we could stop over in XXX for a couple hours for lunch and for you to look around."
Here, an instructor and student can do a multi-day cross country together, or an aircraft owner can hire an instructor to fly with him or her on a trip, as an extra pilot, without anyone needing a charter licence.
Check your air regulations for the difference between passenger transportation and aerial work, or similar terms.