In the immediate Kielder area there's not much in the way of natural trails due to the ground conditions. Apart from the forest roads you're stuck with the trail-centre routes that have gone in over the last five years or the lakeside way.
Having said that, the lakeside way is a pleasant day out (with cake stops). The Deadwater red route is the best of the new trails, although it's quite hard going for a red due to the eroded sandy/ rocky surface in places. The new blue route is not bad but there's a long forest road climb before you get to the good stuff. I wasn't fussed about the lonesome pine trail.
The cafe at the Castle is worth a visit for the food, especially if the weather's like it is in the NE today.