This is a very simple game.
Someone steps up to deliver a powerpoint/ keynote presentation but has no idea what slides are coming.
They are given a subject and then must simply start speaking.
Try letting students do the presentation in pairs if they are nervous - that way the other one can take over if the momentum dries up.
If you've created the slides in advance - make sure they have the same structure to them - you can tell them this at the start:
These decks each have 8 slides, the presentation will advance every 15 seconds meaning each presentation will be 2 minutes long. The last slide in every case will say "Thank You".
This exercise teach portraying confidence in the face of uncertainty. To get the most out of it you need to make big leaps - confidently introduce the next slide before it arrives, justify everything, don't be phased whatever comes on the screen.
If you have Keynote, I’ve created 14 presentations which run as described above. You can find them here.