I'd like to hear them too.
Suppose your church hosts a Sunday afternoon picnic. They bring blankets and food, spread them under a tree, and then give thanks to God.
The only thing necessary for a Jehovah Witness to consider picnics a "God dishonoring practice" and/or "pagan ritual" would be a Watchtower illustration showing tree worshipers with a blanket who may at one time feasted under a tree. To show how this works I randomly grabbed an illustration of Druids:
For good measure I throw in the Norse god Idun, plucking apples from something that suspiciously looks like a picnic basket:
When we look at the origin of the French word "picnic" we see "pic" as most likely coming from the word "piquer" meaning "to pick" (think Idun) and the German word "nique" meaning "worthless thing" which is all we need to show the "real" worldly reason "Christendom" has picnics!
Of course, If I could draw like Watchtower artists, I could do a much better job at conveying this thought through illustration.
But it doesn't matter that Druids and the worship of trees or Idun are the farthest thing from the minds of Christians on a Sunday afternoon picnic. In the mind of a JW, your church is now participating in a pagan practice, spiritually feasting at the "table of demons" every time they spread a blanket, and our refusal to acknowledge this "fact" means that we and everyone at our church will have a meteor headed our way during Armageddon:
In all fairness to the JW's they haven't forbidden picnics yet, but I think I've fairly illustrated their thinking and rationale. No critical thinking is necessary when it's all laid out, illustrated, and guided through the pages of the Watchtower. It's a relatively simple exercise that for many of the church's critics has become persuasive.