As per title...

1. If I drive to Hatyai from Malaysia, what documents I need to bring, and what I need to be aware of?

Your car geran, purchase insurance with the geran, last time can photocopy now not sure
2. Any recommended hotels?

I always prefer VL, or Lee Garden (better security, but need tap access of the girls to come in)

3. What to do in Hatyai?

Massage, cheong, Hatyai marathon, water festival see which event you go la....
4. From KL to Hatyai, what are some good place to have a break, and to makan?

Usually I'll stop and RnR

Thanks in advance