Have a seminar at WDW's Swan which is booked up. What is in walking distance? Boardwalk?

The Dolphin.

That too is full. Any other suggestions?

Any hotel on property can be accessed via the Disney bus system. Yes, you might have to wait 15 minutes, but it isn't too bad really. The Board Walk resorts are fairly close but not walkable. Your price range will probably determine the hotel. We have stayed in many and they are all clean, keep with the theme and enjoyable. We like Port Orleans because we like to hear BOB in the bar. We liked the cheap end with POP Century just because of the nostalgia, The Wilderness Lodge is majestic, etc. You have to decide I guess. Buses go everywhere.

The Yacht and Beach Club is walkable. The Boardwalk Hotel is also walkable. Each is maybe a five minute walk from the Swan/Dolphin.

Oh, yes, I believe the boat can also take you over to those which is a nice short 5 minute ride.