My piggies have responded incredibly well to fresh green treats - parsley in a pot so that they can eat it right down to the soil and make a lovely mess! - posh salad of washed mixed leaves (which is too expensive for everyday chomping) - edible flowers (I gave nasturtiums so I'm not sure what's around at this time of year.
Also good are unusual play toys they don't normally get. Mine adore cardboard boxes and those big paper sacks, but they do wee in them so they do have a shelf life as it were.
Mine also love certain fruits such as melon, but they are also expensive. So a fruit salad portion perhaps.
I have tried posh food but they turned their noses up at it, dried veg and chewy things. Mine do like alfalfa slims though.
Also - sing to them. They like that. They think you are trying to communicate in their sort of style! So sing happy birthday in a soft sort of voice, 'coo' it, and they'll like that!
Happy birthday to your piggies from my piggies, mine have four separate birthdays a year and they all get the treats each time....spoilt? Yup!