I was there 6 months ago - had to get off the train from Bangkok to get a transfer over to the coast for a ferry to Koh Lipe - and had a look around the town for an hour or two.
it looked a bit of a shithole, to be honest. I wasn't there long enough to give any advice beyond going by my superficial impression the best thing to do in Hat Yai is to get the fuck out of it.