It doesn't get much easier than eating a lot of fruit for breakfast. If you have a moment you can make a fruit salad or a smoothie, but in a pinch you can just eat a bunch of fruit whole. Apples, oranges, grapes and grapefruits are all favorites. Bananas are one of your best and simplest choices too. Eating whole fruit starts you off with a lot of clean energy to burn during your day, and the sugars won't make you crash like refined and processed sugars. Not only that, but there's nothing simpler then eating a few pieces of fruit. Apples and grapes take literally no preparation beyond washing. Oranges and grapefruits just need to be sliced a few times. Bananas just need to be peeled. You can do this in your house while preparing other parts of your day, or you can eat them on your commute to work. If you want to eat fruit for breakfast, then just keep your house stocked and you will have NO excuses for stopping and getting a few donuts instead.
If you like cereal and are already used to having it for breakfast, it's really simple to just buy healthier cereal and substitute it for your usual start of sugary puffed junk food. Make no mistake- there is no difference between starting your day with a snickers bar and starting your day with more processed breakfast cereals. Ditch the stuff that even kids should know better about and get something made from whole grains and dried fruit. It can come in a box, or you can buy it in bulk. This is not only healthier, but it's probably cheaper, than whatever junk you normally mix with your milk.
The same holds true for oatmeal. Though they take a little bit more preparation than pouring a bowl of cereal, eating breakfast porridges like oatmeal is even healthier, and much much cheaper. Just make sure you don't get the instant stuff, and instead get the steel cut kind that you need to flavor yourself. A little syrup or brown sugar is a good way to sweeten the bowl, though fruit is even better. Mix in a little ground flax seed for some Omega-3s, and there are few healthier and more affordable ways to start your day. And chances are that great mindset will carry through for the rest of your meal choices.