I don't know why you think you cannot talk about the truth not being the truth, but It's usually more effective to ask questions designed to get them to think rather than telling them things. For example, point out the dew claws on a dog or cat and ask them why they think God put them there, since they are functionalluly useless.That way you are not criticising the religion, just pointing out things that creationism doesn't explain. There are lots of things like that. Redundant DNA, the fact that we share so much DNA with chimpanzees, etc. Get a subscription to National Geograpic if they are older, it will expose them to science.
It might be good to also expose them to the beliefs of other religions. Most JWs know nothing if the beliefs of others, it adds to the illusion that they are special. Go to a United Church of Christ (UCC) service, it's way more interesting and fun than the dry boring meetings. The idea is not to get them interested in another religion, but to expose them to the idea that there is more than one way to be spiritual, the JWs are not the only way, or even the best way.