You're trying rather late in the game. The popular restaurants were snapped up long ago, and at this stage it's going to be difficult to find alternatives inside the theme parks. I agree with trying the resorts - some resort restaurants are better than anything inside the parks, and are very close to parks as well.
If you are not on a dining plan, you can try walking up. You would almost certainly be turned away from the ultra-popular places: Le Cellier, Cinderella's Royal Table, Chef Mickey's, Ohana dinner, California Grill. Long waits are likely at park restaurants, character meals, and a bit less than ultra-popular places like Boma. I also agree with trying online and seeing what types of alternatives come up. Or try restaurants that don't accept the dining plan (mostly at Swan/Dolphin and Downtown Disney)