I went to WDW in 2012 for 3 vacations; 2 of which were 4th of July week and Christmas/New Years week. I do not like planning ahead.
I did walkups during these 2 weeks at Citricos, Narcoosees, Sanaa, GF Afternoon Tea, Marrakesh, Kouzzina, Il Mulino, bluezoo, Brown Derby.
During non peak vacations, I have done walkups to many restaurants. I only do one table service a day and I usually try for lunch around 2pm or dinner at 8-9pm. These are always for a party of 2 or 3. I do make ADRs for character breakfasts and V&A but prefer to fly by the seat of my pants on the others.