Any advice on the best way to get to the Malaysian border (Sa Dao?) from Hat Yai, i.e. private taxi or visa run agent (if so where from)? I just need to cross the border and come back as I have a re-entry visa. Is it as simple as just crossing the border and coming back again 5 minutes later? If so can I expect to be back in Hat Yai with a new 90-day stamp in a matter of hours?

Thank you.

This is several years old but believe still the same.  Next to Kings Hotel in Hat Yai is a cafe that taxi drivers hang out in (they have signboard outside and sometimes sit on sidewalk).  They can take you to Penang or just do a border run.  You have to bargain price.  They take to Thai side and you process out, drive you to Malay side (but not through) and you walk up to window and get Malay stamp (no immigration card needed if you tell them just a border crossing) and then walk around to the exit area, get stamped out and back into car for drive back to Thai entry and new 90 stay.  If you do in morning you can be back in time for the buffet lunch at Lee Gardens Plaza.  If you fly to Hat Yai the taxi drivers at airport will also make a border run but may want too much.