carrots and peas you can give them too. VItamin C, and the carrots enhance color. I don't thinkg your suppose to feed the guppies garlic, but soak their food in it, its just for taste. Guppies will eat and nibble on just about anything, just don't spoil your water.
I haven't tried garlic... I may have to give it a go tonight.
I've fed all my fish peas. Just make sure you break up the peas into small pieces. While guppies CAN work their way through a whole pea, they will argue over it.
I've also found my fish like cucumber. I tried to bait snails with it. (The advice being put the cucumber in the tank over night, the snails will move in on it, then you can just lift them out.) It didn't work for the snails (Lesson of that day, cucumber floats. If it floats the snails can't get to it...) but the fish LOVED it.