There are many excellent suggestions here already. We have been making ESPN our lunch stop on day of arrival (our home is BWV) and find it pretty decent. We usually end up with some leftovers in the fridge because of the large portions. Although I've read mixed reports on Big River Grille, we have found it to be okay as well.
Lighter luncheon fare includes food at the pools and bratwurst (with sauerkraut and mustard, please) from the hot dog vendor on the BW. You add your own kraut and mustard, so I get it just the way I like it, heavy on the kraut. Not Wisconsin brats, but pretty good. There is a combo price for hot dog/fries/soft drink or brat/fries/soft drink which makes this an excellent value by WDW standards.
In addition to Tubbi's at the Dolphin, don't forget the Dolphin Fountain Ice Cream Shop. They have sandwiches, burgers, etc. Good quality ice cream as well. It's a toss up between the Dolphin and Beaches and Cream, but I think the Dolphin has more seating room, so it can be quicker. Plus they have been giving a 20% discount.
The Fish & Chips vendor adjacent to the Rose & Crown Pub in Epcot is pretty handy, since practically there when you enter Epcot at the International Gateway.
Finally, there are usually an accumulation of dinner leftovers from the previous days' dinners as we like to enjoy multicourse dining (appetizer, entree, dessert) when on vacation and the amount of food is then too much for the two of us to eat at one sitting. Having a fridge back in the room makes it convenient to bring leftovers back for reheating/eating later. Last trip, we didn't manage of consume all of our leftovers and had to trash them when we vacated the room, but it's nice to have the option.
Ralph
