Not gnarly at all, but you can pick up NCR 14 at Hart and have a pleasant potter North to where it meets NCR 1 and up through South Hetton, Murton, Seaton and into Ryhope. There are some nice bridleways that spur off NCR 1 at Murton and Seaton. Alternatively, bimble up NCR 14 from Hart and pick up NCR 1 and head South to Thorpe Thewles for a lovely cream scone and a cuppa 😀
There’s also the coastal path (cross the line at the bridge at Hart) which is a nice bit of cross country type stuff up to Crimdon, through the caravan park (Butlitz!) into Blackhall.
Dalton Piercy has some nice bits and bobs as well.
edit- and there’s the BMX track at Summerhill for being rad and stuff.