Here's what I post in my blocks area. I talk about it with the kids in language they understand but the posters are really for other adults (principal, volunteers, etc.) to tell about what skills we're working on.
Thankfully our principal encourages us to have blocks, housekeeping, sand & water - all those developmentally appropriate things for our little ones!
Playing with blocks can lead to some great dev. appropriate questions you can ask the children that will also elicit some higher order thinking skills:
Is there another way you could try it?
Would it help to have some ___?
How did you decide on your design?
Tell me about your creation.
What could you do with these things?
Can you change it? How?
Why do you think your plan didn't work?
What would happen if I ___? Why?
I found a great article on why puzzles are good for children:
http://toys.about.com/od/learningtoys/f/whypuzzles.htm