Don't say anything about the wtbts. Say something like, "you look tired!" with a really friendly voice and a smile. Or, "how's the weather for you? What do you do when the weather is too bad to be out here?"
Last time I saw some JWs with the little cart, a couple of women, I smiled and commented on the view (it was at the beach.) "What a wonderful way to spend the day!" I chatted inconsequentially for a few minutes, until they relaxed a bit. I said my mother had been a JW her whole life, but I'm just a cheerful old apostate. Then left before they could get themselves worked up.