we usually don't make a lot of ADR's and always have no problem w/lunch walkups at Olivia's, Yacht Club Galley, ESPN, and any DTD restaurant.
For dinner, try calling the day of to see if you can get an ADR. Barring that, try any non-monorail/Epcot/AKL resort hotel restaurant (Boatwrights, Shutters, Turf Club, Olivia's). You will also be able to get into a DTD restaurant but plan accordingly (put your name in and be prepared to walk around for awhile).
Or do what we do quite a lot, eat at the bar. Most places will let you do so w/out an ADR, esp. at DTD. The only place we were told no at WDW (of places we actually asked) was Flying Fish but I really think I've seen other people do so in the past. They really were rude about it too.