Run by the National Trust and located near Royston (it takes us 52 mins from Fleetville) Wimpole Estate is one of our favourite places to go.
There's a lovely little working farm (Wimpole Home Farm) where you can get up close to rare breed sheep, pigs and horses amongst other things. It has a great play area surrounded by picnic tables where you can relax whilst the children run off some steam and interesting old farm buildings to look around.
There's also Wimpole Hall, the parklands the walled garden and the beautiful formal gardens to explore and for eating and drinking there's the Old Rectory Restaurant and the Stable Yard Cafe. If you're a National Trust member then it's totally free to visit!