I would want a proper guided tour, with personal safety guaranteed. Most fictional worlds are not your nice oh-so-safe twee universes, they're flaming dangerous. And in the case of Narnia the risk of hypothermia is extremely high! (It also pays not to annoy any talking lions you happen to come across. The Narnia stories do indicate this is unlikely to go down well).
So where would you visit? I've always liked the sound of Middle Earth. I love The Lord of the Rings and thought the Peter Jackson films magnificent. I loved the hobbit homes. Mind, being under 5' tall, I feel I have a natural affinity here though I can confirm I don't have hairy feet!
I wouldn't want to go to Mordor (twinned with whatever ugly modern town you can think of!).
I quite like the idea of visiting Gallifrey (no prizes for guessing who my guided tour would be led by there!). I would visit Narnia, suitably well wrapped up (though until the thaw did come I'd want to be able to have a constant supply of hot chocolate on the go).
So where would you visit?