The talk can be entirely congratulatory, or it can contain a bit of actual mathematics ... any alternative suggestions?

How about some history of mathematics?

It could be structured around a few notable mathematicians throughout history (please try to include women). Or around kinds of problems that concerned people of different eras: are all numbers rational, how to solve polynomial equations, what does infinity mean, what does computable mean, ..., ending with what you feel is a central problem today.

To ensure older students haven't seen it all before, specific and obscure may be better, e.g. spending more time on anecdotes from Ramanujan's life or Erdos' than on explaining their importance.