How should I respond to someone's research "elevator pitch"?

what questions are best to ask them to keep the conversation going after they have given this spiel when I am a novice in the field?

You're overthinking this. Pretend it's a conversation between human beings: do you have a question about what they said? Ask it. Don't you have any question or you find their research topic utterly boring? Change topic of conversation. Exactly as you would do with anyone else.

Summer schools frequently have students with different backgrounds and from different countries; you will be all in the same situation, and for most of you it will be a new place: there will certainly be things around you that can be subject of conversation.

As for your last question:

I don't feel that I know my own topic well enough yet to explain it properly in return- so what could I say instead?

Tell the truth: That you're going to start your PhD on topic X, that you like it for reason Y, but you still don't know much about it because you just started and that you will be super-happy if you can meet someone there working on the same topic.

The "best" response I ever heard was:

I understand. By the way, what do you think about [my great research topic]?"

Substitute [my great research topic] with you subfield of cosmology.