There is aluminum service drop cable that can be used.
http://buyawg.thomasnet.com/viewite...m-conductors-aac-neutral-messenger?&forward=1
Table 300.5 requires only 12" burial for 120 Volt circuit with GFCI protection, and you can bore under a sidewalk. I would put THWN in a 1/2" PVC conduit. You need the GFCI for the carport anyway.
You need only 6" if it is rigid or intermediate conduit. A few $ for 30 ft of 1/2" intermediate conduit would be worth the saving for the extra 6" of digging. Since you are doing it for a relative they should be willing to pay for the conduit if you are doing the digging. If they are doing the digging they can make the choice.