Without a doubt, the location would be one of the Parks. WDW would be my choice because I'm most familiar with that Park versus the others. If Walt wanted a different location, that would most certainly be OK!
For conversation, I'd like to give him a brief summary of the current state of the Parks and the Disney company, because I imagine he'd be interested in hearing about it. I'd then like to hear what he thought things would be like versus what they are like now. If there was time after that, I'd love to hear him talk about his vision for EPCOT (the city, not the Park), and then his vision for space exploration and where he would hope things are today.