Just coming on to say that we've been doign this for a few years actually. I think as others have pointed out, it's important to remember this is about the gut biome and variety. And I'll tell you one huge benefit for us - the post-toilet-smell is a LOT better!!! 
A few tips I've worked in:
All salads always have extra in them - I buy mixed bags of leaves. That can be pricey so I might buy one iceberg lettuce and then a mixed bag rather than two different mixed bags. I also make a big effort to add something extra to salads - seeds/nuts, avocado, spring onion etc. I always buy peppers in bags of three - mixed colours - so that's good too.
Similarly, I often buy a single bowl of mixed stir fry veg, using veg that I don't usually buy, and then add my usual veg to it when making stir fry.
Lentils are a great way option to add to stews/curries/sauces. Chickpeas and beans too if you have less fussy eaters (red lentils just disappear, so that works well).
Smoothies using frozen fruit are brilliant as a bag of mixed berries can be 3 or 4. I also always have flaxseeds in the house which I add to smoothies, and I always add fresh apple, banana and also oats.
I make homemade granola weekly now. That gives us a wide range of nuts and seeds, alongside the obvious oats.
Think about variety in types of food - by choosing to cook Asian one day, mexican the next, European the day after, you're upping your range of herbs and spices, even if they're just dried. I routinely use around 10 different types of dried herbs or spices every week, as well as fresh ginger, garlic, chilli routinely.
I also usually buy one "extra" fruit a week. So a watermelon one week, or bag of peaches another etc.