I wouldn't bother with piggy treats really. Things like honey and oats are not really the healthiest foods for them, and it sounds yummy to us but I think they'd prefer just a nice bit of lettuce or some dandelions or grass as a treat.
Same with some shop bought treats, that have things like seeds and honey, they're very high in calories, and some fresh veg is a better treat.

Although you can buy some treats that are like dried herbs and stuff, which would be good too.
For a birthday cake then I would do something like arrange veg in a cake shape, maybe with some carrot sticks as candles

Ok, it sounds silly, but could be colourful and fun.