"Dogwood," I said, and then, "cowslip."
There were four of us in the car; we were on the way to pick ollaliberries, kind of a silly word, and I was thinking about names of plants.
"Catnip," said Sarah, picking up the theme without having to have it explained.
Arthur mentioned "lambsquarters"; Ananda tossed in "foxglove." Various of us brought up chickweed, lousewort, and pigweed. The game was on.
They were all more knowledgeable naturalists than I, and came up with many that I'd never heard of (as well as several I just hadn't thought of). We continued playing, on and off, for the rest of the day, whenever there was a lull in conversation or someone thought of a particularly good plant. It's a nice on-the-road game.
Try subcategories of plants. Banes (start with wolfsbane); worts; berries; flowers; trees. And categories of animals: mammals in general are easy, but how about rodents? Or insects? Reptiles and amphibians are particularly tough; we only came up with two or three of each.
You can range further afield if you get bored: animals at the ends of plant names; people's names or professions (from Queen Anne's lace to miner's lettuce); body parts (bloodwort). You can even stretch the concept to plant products that only accidentally contain animal names, such as "rattan."
And if you want to get really silly, you can make up your own plant names. I was especially proud of the word "leechbelly"—it sounds like it ought to be the name of a particularly unfortunate plant with blood-colored leaves, doesn't it?
As usual for word games I play, the point here was not to keep score but to have fun. There may have been a small amount of competition—trying to top each other's words, to find more interesting, more amusing, or more obscure words than the others. But if you want to play this really competitively, you can try to come up with (and write down) as many words in the given category as possible, with a time limit. Ignore all the words more than one person came up with, and count the unique ones. Highest score wins. (These rules are more or less derived from the rules for the game of Boggle(TM).)