What could you give a 40 minute presentation on with absolutely no preparation in advance? (Spur of the moment)

I could teach a Kempo class.

I could give a briefing on the fishing along Florida's Gulf coast.

There are numerous historical references I could speak about at length.

Nothing, as I'm way too shy...unless, it's just my cat as the audience?! :O

Forty minutes is a long, damn time..................hmmm........................................................can't think of anything to go that long.

Basics of building a house
Remodeling a kitchen, bathroom
Building a deck
Change brakes on a car
Properties of gold, silver or platinum
Choosing a diamond
Investing in gemstones

ANYTHING ... As long as I'm not talking to anyone with knowledge of the field.  LOL

If it was a small relaxed crowd, I could talk about art and painting.

I don't think I could give a 40 minute presentation on anything. I remember having to do 3-5 minutes speeches in college and that seemed like an eternity.

My favorite TV show and why you should watch it! :D just kidding (or not)
A few Space topics (black holes,wormholes,..)
History of civilization,or history of few places and events
Some social issues
Few of heart diseases and their treatment

I've volunteered at hospitals and places like that before to assist those who are going through the process of getting a prosthetic. I've talked to a lot of children. Since I went through that and many of the doctors know me quite well by now, they'll let me talk to some of their younger patients. It seems to help some of the kids if they talk to someone who's done it all before.

I could also talk about why Skyrim is the best video game in the world and why everyone should play it! :P

I could on cross stitch.

How the Fair Labor Standards Act applies to federal employees.    And I fgaurantee that the 40 minutes will seem like 4 hours. 

I could teach a basic cooking class. How to correctly use certain kitchen tools and appliances.

Personal, personnel, document, physical security.

Women's Self Defense class

Making the perfect Waffle

The Concept of Secular Morality