Well... if you'd like to start with a joke, when you're ready to get started, stand in front of them and say '
"Hajime ni, Rajio Taisou Shimashou!" It would probably get a laugh. Work places and schools do Stretching exercises every morning, and they just listen and do it to a recorded voice, so this is basically saying 'To begin, let's do Radio Exercises!'. They'll know exactly what you're talking about and laugh, I'm positive. If only I could help with pronounciations on here...
Something you should try not to do is point with one finger if you have to point at someone, use your whole hand. They won't storm out offended at all, but in the Japanese culture you don't point.
I'm trying to think about what else would be helpful, but that's really all that is coming to mind. Also, they'll most likely introduce themselves by their last names, and it's appropriate to add 'san' like Miyagi San to the end of their name, it's basically 'Mr.' But remember, it goes after the name. Mine would be 'Jenkins San'. That's 'San', and is 'Sawn', not like Sans Serif.
It's hard to type teach a language!
Actually... if you really want to use that first joke, I can make you a graphic for your keynote that will say that in Japanese, and then you can just say it out in English with that behind you...
I lived in Japan for several years, and can probably help you out if you have any other questions.