I sometimes stick the core of a haylage bale in for mine - they love diggin in it, chewing it, lying on it etc
They love playing chase with me but I have to be careful now they're getting so big!!
A swede on the end of a rope is fun to swing around for them.
In the summer I froze apple juice in freezer bags and gave them giant ice cubes.
Failing that I let them out to play in the muck heap ... their favourite!!