From Peterborough Nene Valley there is a foot/cycle path, running the length of the line to Orton Mere station. There are a few good spots along that section, the best of which is probably at Longville Junction, where the main line connection joins and the line runs as parallel single tracks into Orton Mere station. The shot from the south side, looking towards Orton Mere is a particular favorite of mine.
Beyond Orton Mere the foot/cycle path continues but the line beyond the foot crossing is fully obscured by a thick band of trees. At Yacht Club Crossing the cycle/footpath crosses the railway, which is also a good spot. From there to Ferry Meadows station (and beyond) there are not any particularly good spots until you reach Lynch Bridge, where the railway crosses the River Nene for the first time. All of this is in the Nene Park, which has numerous walking routes that make most of the railway accessible.
After Lynch Bridge there are a few more locations, such as Castor, but these really need vehicle access as they are a good walk from Ferry Meadows station. On the approach to Wansford the line crosses the River Nene again and there are a number of good spots alongside the river and on the bridge, all accessible from Wansford station.
From Wansford the line is in a cutting, runs under the A1 and through a tunnel, emerging just short of Yarwell Junction station. The latter is adjacent to a nature reserve and there are a few good spots where you can get pictures of loco run-rounds - although on gala weekends it is normal to attach a loco at the opposite end of the train and detach the trailing loco when the train gets back to Wansford.
HTH