i got mine a bottle of good supermarket wine (~8 pounds) - considering the amount of supervision i got, i think that's fair. one phd in another group at my uni invited his supervisor and the supervisor's family (wife, 2 kids) to a theme park for a day and they all loved it. but he got on amazingly well with his supervisor. i guess it depends on how much your degree is really 'owed' to your supervisor.