Our cheapest "eats" while traveling have been free coconuts in Fiji. I would husk the coconut the day before and chill it in the hotel room mini fridge.

The chilled coconut milk and meat was so good the next morning. ^

Other than that, we've had cheap oatmeal plenty of mornings while traveling to save money and also time. I love having a quick meal of oatmeal in the room and getting out to the beach / park / hike early in the morning.