I have to tell you that I think the days of walking up and getting a seat are almost gone....with the dining plan, it has become increasingly difficult to do that.
Are you staying on property, if so, you can go to the guest services at any resort and have them check for seating availability at any restaurant for any day. I would suggest that....you can still try to do walk ups but I have experienced being turned away at the resort restaurants just trying to walk up never mind the highly patronized restaurants in the parks.