Ooh, that's a really interesting question, Mountie! Props to your dad for thinking of it.

I think I'd most like to spend the day with Professor Kirke. He seems like a really smart, well-read and quirky old man, and I would love to talk to him all about his adventures as a boy and all of his knowledge and opinions relating to the world of Narnia. Heck, it would be so neat just to be able to spend a day sitting in on one of those Friends of Narnia meetings!

It would also be awesome to talk with Aslan, but I have a feeling I'd be too in awe to say very much.

It's a very hard decision, and I'm not sure I could pick just one person to meet....
I do really like Lucy, though, and it would be really awesome to get the chance to meet and talk with her. It would be nice to meet her (and her siblings for that matter) when she was all grown up, but at the same time, I'd really like to meet her when she was younger.
I'd also like to meet Puddleglum. I feel like we'd have a lot of interesting conversations. I'm not sure if I'd like him as much in real life as I do in the book, but I'm sure it would be fine for one day.
And yes, I'd also like to meet Aslan.

I guess that was three, not one, but it was really hard to pick.

I think it'd be interesting to visit with Trumpkin for a day--he saw some of the most interesting bits of Narnian history, and to hear what he remembered about the Pevensies and Caspain and meeting Aslan... and even second hand stories about the Dawn Treader would be pretty neat.

Aslan if I wouldn't be too in awe to say anything!

For human characters, I think Lucy. But I would love to just be adopted as a fifth Pevensie, to be honest:)