Write a list of vocabulary words on index cards. Divide the stack equally among the players and have them place their cards face-down in front of them.  Each player turns over their first card. The card that comes first alphabetically wins IF that player can provide an acceptable definition of the word. If so, the player takes the cards from the other players. If not, the player to the right has a chance to provide a definition and steal all the cards. If two or more players lay down cards that begin with the same letter, the player who first yells "WAR!", wins. When all the cards are gone, the player  with the most collected cards wins.

The term "nature study" can sound a bit intimidating, especially if you're not a science buff, but it really shouldn't be. Instead of focusing on trying to teach specific facts, focus on teaching your children to become AWARE of the world around them. Then, simply follow their curiosity. You'll be amazed at what you end up learning.