Airplane food - even on Air Force One - can get tiresome, so White House spokesman Marlin Fitzwater tried to perk things up by planning the dinner for passengers traveling Saturday with President Reagan. "I've had it with those wimpy tuna salads on the half shell," said Fitzwater of his foray into menu design: tossed salad, beef Wellington, buttered new potatoes, broccoli and cauliflower florets, rolls and butter and apple pie. "See that, folks?" Fitzwater said with a flourish. "No spinach, no turnips or any of that foolishness."