Quinoa flakes (like oatmeal) and chia seed pudding are great, healthy fillers for snacks or lunch time sides. You can add fruit to either or have them on their own. Quinoa flakes are similar to oats - they're super quick to make and versatile, mashed banana and cinnamon are a favourite with the kids here. My personal favourite for breakfast is quinoa flakes with pureed strawberries, a few chopped walnuts and dark chocolate chips and a sprinkling of unsweetened coconut on top.
Something we've found too is that our grocery store has a clearance rack for fruit & veggies, we find a lot of great produce marked down by 50%, we got bags of organic apples and pears for a couple dollars, they lasted in our fridge for a couple weeks while we used them up. Bananas we pay $0.50 for a bunch and usually still have to let them ripen. We check the clearance rack first now to see what's there!