If the speaker fails to turn up we do things like match the common name to the botanical name type quiz. as well as images of seeds- what does it become?
seed heads- guess the plant etc.

Years ago we did the alphabet game. for each letter of the alphabet name a plant. common names is easy but botanical names can be more of a challenge.
Then there is the childrens favourite that I have used at Xmas events.
When I went to the garden centre...
first person says when... garden centre I bought [name a plant beginning with A] Achillea,
2nd person repeats the first persons statement and plant and adds a plant beginning with B
eg When I went to the garden centre I bought an Achillea and a Brunnera.
3rd adds a plant beginning with C etc
the winner is the one who remembers the most plants without forgetting.

Still play this one with my 15yrs old. keeps the faculties sharp haha.

If you mean questions and answer games then my first thought was plant identification.
Leaves of trees and shrubs, some attached to small branches, flowers, common and unusual but all from your local area.....Be sure you are right though as someone is bound to query it!

Or a sort of treasure hunt, send them out and when they find the right tree or plant the next clue sends them on further......have a time limit though or you may never get some of tbem back. :0)

I like that idea seaburn, sometimes if I can't sleep I try the alphabetical plant list.......

Croqouet, It's a game any age group can play, its easy on the limbs, and can be played at a nice steady speed.

And good fun if you 'cheat' lol!

Victorian ladies would hide the ball with their long frocks and move it to a better position......or so I read.....

Croquet always was a good game for scoundrels!

Painful though if you put your foot on the ball for a "roquet" is it called?
And wallop your ankle by mistake. :0)

Ouch!

Thanks all for your great ideas:)