At 6pm? A walk-up isn't likely.
Right when they open? You've got a better shot. My grandparents wanted to eat at OHana's in March and I tried and tried and tried and tried for an ADR for them, no dice. They decided they'd wing it, got there right at 5, they were told it might be up to an hour and a half wait, so they grabbed seats at the bar. 30 minutes later they had a table.
So it's not
impossible, but it's not likely, especially the later you go.
As for Hard Rock or Planet Hollywood, no need for ADRs. Also, these places are good for walk-ups: Spoodles (if it's still open), ESPN Zone, The Brewery Place on the Boardwalk that I can never remember the proper name for

, Beaches and Cream, Jiko, Artist Point, Yachtsman, Kona, Grand Floridian Cafe, Marakesh, and places where you can sit at the counter or bar...like California Grill, Jiko, Flying Fish, 50s Prime Time...and there might be some more. But on those with a bar/counter, be prepared to just sit at the bar and not have an actual table.