ok weather a bit cold for my first idea. my nephew loves ducks so for his birthday we all just went to a park had a picnic by the ducks and let the kids feed them but his birthday is august so was warmer.
why not go to a farm not sure where you live but we go to ash end house childrens farm, they do a birthday package where they get to feed the animals and pet the smaller ones.
or the zoo
or safari park