The general rule of thumb with non-human animals is to avoid human food, which tends to be high in pretty much everything unhealthy. Cucumbers and peas should be fine in moderation, though the species is carnivorous (well, I suppose more like "insectivorous" for the most part since they're so small) and lacks a dietary need for it.
I would suggest, in addition to some of the insect foods people have suggested, varying the pellets as well. Different pellets come with different fat and protien contents, different levels of softness/hardness, and in different sizes. One of the main rules of animal nutrition is that in most cases, variety is indeed the spice of life. Feeding one pellet brand, even a high quality pellet like Hikari, is probably not providing 100% complete nutrition.
I personally feed a mix of BioBlend (betta), Hikari, and tadpole pellets myself. The tadpole pellets are soft with a higher fat and protien content. BioBlend is also softer than Hikari with moderately higher protien and fat, and more phosphorus. All three a very good diets for a carnivore.
If soaked, flakes are probably pretty safe as an occasional treat. I have some betta flakes myself, though I haven't fed them in ages since my boys don't much like them.
As for frozen and live foods, blood worms, brine shrimp, fruit flies, and neo-natal (smaller than pinhead) crickets are all great. I've heard of people feeding frozen liver and beef heart, but I have to question feeding chicken and cow to an animal who wouldn't eat anything bigger than a fly in the wild. Since livers are the main filter for toxins, I'd never feed a fish (or a person, for that matter) liver considering the amounts of hormones, anti-biotics, and pesticides our livestock are exposed to.
Daphnia are also good in moderation (they are a very dry food). I have a large container of them that I got months ago, and there is still barely a dent in it. It is definately a good buy considering how long it lasts.
