I make a batch of overnight oats, enough for about 3 portions, & store it in the fridge. I usually eat it for breakfast but it would be good for a snack too. Basic recipe:
125g / 1.5 cups thick milled porridge oats
1 heaped tbsp chia seeds
1 orange
2 apples - granny smith are good for this
3 tbsp protein powder- I use a plain whey one
Spices if you like - about a tsp (total) of ginger & cinnamon, or 1/2 tsp vanilla
Peel the orange with a knife. Remove all the membranes & seeds, chop the flesh. Don't waste any juice. Tip into a plastic or glass lidded container. Wash, core, & coarsely grate the apples, tip them in as you do them & mix into the orange, it stops them oxidising. Add the dry ingredients. Add enough water to make a slightly soggy mixture. Mix thoroughly. Leave at least 4 hours, preferably overnight. I eat it with Greek yoghurt & fruit - frozen is easy & cheap, mango or berries, or whatever us fresh from the fruit bowl.
You can leave out the protein powder, chia seeds if you want, you'll need less water. You could add chopped nuts, dried fruit (not banana chips as the texture when rehydrated is vile IMHO)
I also like chia pudding. I make chocolate chia pudding & have some in the fridge most of the time. It's very nice with frozen cherries, a bit like a Black Forest frozen dessert with less guilt. My diet is really difficult & having stuff ready prepared helps a lot.