Bircher muesli/overnight oats would definitely make a better breakfast. Just soak the oats in milk (or a milk substitute, almond or coconut milk are delicious) overnight to soften, then you can add whatever you fancy in the morning - yoghurt, grated apple, fresh or frozen fruit, nuts, spices. Cherry and almond milk is a good combination.
Fruit salad doesn't keep as well as fresh fruit so it is better to make it just before you eat it. Cutting the fruit speeds up decomposition and some loss of vitamins, particularly antioxidants, C, A and E. Frozen fruit doesn't always look pretty but freezing preserves the vitamins so it can be better than fresh.
Berries, cherries, nectarines and peaches are in season and at their best at the moment. One of my favourite combinations (although it is a bit more autumnal) is figs, blackberries, black grapes, pomegranate and coconut, sometimes I add rosewater but maybe not for breakfast. Peaches with a raspberry coulis (a word rarely heard this century
) is a good "tinned" option out of season, if you are on a budget.
The "eat a rainbow" advice for fruit and veg is a good principle. Different coloured fruit tend to have different nutrients.