Don’t be overly affectionate in public. This has changed in recent years and younger Thai couples can be seen holding hands, but snogging your boyfriend or girlfriend in the middle of the shopping mall won’t win you too many friends. As with many things, Thais know that behaviour in the West is different to Thailand so you won’t be chased out of town for holding hands with your partner, but resist the temptation to do so inside temple grounds.
Don’t sunbathe nude. This is offensive to most Thai people although nobody is likely to say anything to you if you do so.